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docs/localization/i18n/i18n-formatter-number/src/formatter/formatter.md

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+# NumberFormatter
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+
+## Purpose
+
+Locale-aware number formatting backed by `Intl.NumberFormat`. The package exposes both a class (`NumberFormatter` — construct once with stable options, reuse) and a standalone helper (`formatNumber(value, locale, options?)`). Every call validates options through `NumberFormatterOptionsSchema` (Zod) at the boundary, maps `Intl` `RangeError`s for unsupported locales to `I18N.UNSUPPORTED_LOCALE`, and emits a wall-clock duration sample on the `i18n.format.number.duration_ms` histogram.
+
+A per-`(locale, options-hash)` cache for `Intl.NumberFormat` instances amortises construction cost — important on dashboards rendering hundreds of formatted values per second. Cache reuse increments the `i18n.format.cache_hit{formatter="number"}` counter so consumers can measure cache effectiveness.
+
+## Features
+
+- Class form (`NumberFormatter`) for fixed-options reuse + standalone helper (`formatNumber`) for one-off calls
+- Full `Intl.NumberFormat` option surface: `style: 'decimal' | 'percent' | 'currency'`, `currency`, `currencyDisplay`, fraction digits, integer digits, significant digits, `useGrouping`, `notation` (`standard | scientific | engineering | compact`), `compactDisplay`, `signDisplay`
+- Zod-validated options at construction; fraction-digit inversion (`min > max`) rejected with `field === 'fractionDigits'` context
+- Stable error codes: `I18N.INVALID_FORMATTER_OPTIONS`, `I18N.UNSUPPORTED_LOCALE`
+- Process-wide `Intl.NumberFormat` cache keyed on `(locale, options-hash)`; `clearNumberFormatterCache()` for tests
+- OTel instrumentation: `i18n.format.number.duration_ms` histogram, `i18n.format.cache_hit{formatter="number"}` counter
+
+## Usage: Basic Example
+
+```typescript
+import { formatNumber } from "@suaveplan/i18n-formatter-number";
+
+formatNumber(1234567.89, "en-US", { style: "decimal", maximumFractionDigits: 2 });
+// → "1,234,567.89"
+
+formatNumber(0.255, "en-US", { style: "percent", maximumFractionDigits: 1 });
+// → "25.5%"
+
+formatNumber(42, "en-GB", {
+  style: "currency",
+  currency: "GBP",
+  currencyDisplay: "code",
+});
+// → "GBP 42.00"
+```
+
+## Usage: Advanced Example — Class Form for Repeated Use
+
+```typescript
+import { NumberFormatter } from "@suaveplan/i18n-formatter-number";
+
+const usd = new NumberFormatter({
+  style: "currency",
+  currency: "USD",
+  minimumFractionDigits: 2,
+  maximumFractionDigits: 2,
+});
+
+// Many call sites, single options validation:
+const out = prices.map((price) => usd.format(price, "en-US"));
+```
+
+## API Reference
+
+### `formatNumber(value, locale, options?): string`
+
+Formats `value` per `locale` and `options`. Validates options through Zod, throws `I18nError(I18N.INVALID_FORMATTER_OPTIONS)` on failure. Maps `Intl.NumberFormat`'s `RangeError` for unsupported locales to `I18nError(I18N.UNSUPPORTED_LOCALE)`.
+
+### `class NumberFormatter`
+
+- `constructor(options?: NumberFormatterOptions)` — validates options at construction
+- `format(value: number, locale: string): string` — formats with stored options
+
+### `clearNumberFormatterCache(): void`
+
+Test-only. Drops the process-wide cache so subsequent calls re-construct `Intl.NumberFormat`.
+
+### Types
+
+`NumberFormatterStyle`, `NumberFormatterNotation`, `NumberFormatterCurrencyDisplay`, `NumberFormatterSignDisplay`, `NumberFormatterOptions`, `NumberFormatterCacheKey`.
+
+### `NumberFormatterOptionsSchema`
+
+Zod schema validating the `NumberFormatterOptions` surface. Rejects unknown enum values, out-of-range fraction-digit counts, and the inverted-fraction-digit case (`minimumFractionDigits > maximumFractionDigits`).
+
+## Implementation Notes
+
+The cache key is `${locale}|${JSON.stringify(options)}`. JSON.stringify sorts keys alphabetically when given an object literal in modern engines, so two callers passing the same options in different order share a cache entry. Order-sensitive object construction would defeat the cache; if a consumer needs deterministic ordering, normalise the options shape in the call site.
+
+`style: 'currency'` requires `currency` to be a 3-letter ISO 4217 code. The Zod schema enforces this; bad currency codes either fail Zod (`field === 'currency'`) or fall through to `Intl.NumberFormat` which throws `RangeError` mapped to `UNSUPPORTED_LOCALE`. Most production usage hits the Zod path because callers know their currency codes at compile time.
+
+The `signDisplay: 'negative'` option is part of the ECMA-402 spec and Node 19+ / Bun runtime; older runtimes may reject it via `RangeError`. The error mapping path produces `UNSUPPORTED_LOCALE` even though the failure is option-related — this is the correct user-facing signal because the runtime is what's missing the support.

docs/localization/i18n/i18n-formatter-number/src/helpers/helpers.md

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+# Number Helper Formatters
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+
+## Purpose
+
+Pre-bound shortcuts over `formatNumber` and `Intl.NumberFormat` for the three most-asked number-formatting shapes: percentages, compact notation ("1.2K"), and units of measurement ("12 km/h"). Saves callers from spelling out the full options bag every time.
+
+The percent and compact helpers route through `formatNumber`, so they benefit from its Zod validation and per-(locale, options-hash) cache. `formatUnit` calls `Intl.NumberFormat` directly because `style: 'unit'` is intentionally outside the narrow `NumberFormatterStyle` union.
+
+## Features
+
+- `formatPercent(value, locale, options?)` — `style: 'percent'` shortcut
+- `formatCompact(value, locale, options?)` — `notation: 'compact'` shortcut ("1.2K", "1.2M")
+- `formatUnit(value, unit, locale, options?)` — `style: 'unit'` shortcut for measurement units
+- `unit` is any `Intl.NumberFormat` unit identifier (`'kilometer-per-hour'`, `'celsius'`, `'megabyte'`, ...)
+- `formatUnit` style is `'short' | 'long' | 'narrow'` (default `'short'`)
+
+## Usage: Basic Example
+
+```typescript
+import {
+  formatCompact,
+  formatPercent,
+  formatUnit,
+} from "@suaveplan/i18n-formatter-number/helpers";
+
+formatPercent(0.255, "en-US", { maximumFractionDigits: 1 }); // → "25.5%"
+formatCompact(1234, "en-US");                                // → "1.2K"
+formatUnit(12, "kilometer-per-hour", "en-US");               // → "12 km/h"
+formatUnit(20, "celsius", "en-US", { style: "long" });       // → "20 degrees Celsius"
+```
+
+## Usage: Advanced Example — Stat Block
+
+```typescript
+import {
+  formatCompact,
+  formatPercent,
+  formatUnit,
+} from "@suaveplan/i18n-formatter-number/helpers";
+
+const stats = {
+  cpuLoad: formatPercent(load, locale),
+  memUsed: formatUnit(memBytes, "megabyte", locale, { style: "long" }),
+  reqsPerMinute: formatCompact(reqs, locale),
+};
+```
+
+## API Reference
+
+### `formatPercent(value, locale, options?)`
+
+Pre-bound `formatNumber({ style: 'percent', ...options })`. Note that `value` is the proportion (0.25 → "25%"), not the percentage (25).
+
+### `formatCompact(value, locale, options?)`
+
+Pre-bound `formatNumber({ notation: 'compact', ...options })`. Defaults to `compactDisplay: 'short'` from `Intl.NumberFormat`. Override via `options.compactDisplay`.
+
+### `formatUnit(value, unit, locale, options?)`
+
+Locale-aware unit formatting. The `unit` is one of `Intl.NumberFormat`'s sanctioned units; the [Intl.NumberFormat units list](https://tc39.es/ecma402/#table-sanctioned-single-unit-identifiers) is authoritative. Composite units use `-per-` joiner (`'meter-per-second'`).
+
+## Implementation Notes
+
+`formatUnit` does NOT route through `formatNumber` because adding `style: 'unit'` would require widening `NumberFormatterStyle` and `NumberFormatterOptions` to include the unit-specific options. Keeping `formatUnit` as a thin direct wrapper preserves the `formatNumber` surface's stability.
+
+Compact notation uses the runtime's locale-specific compact patterns. For en-US that's `'1.2K'`, `'1.2M'`, `'1.2B'`; other locales use locale-correct abbreviations (e.g. `'1,2 万'` for ja-JP).

docs/localization/i18n/i18n-formatter-number/src/observability/observability.md

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+# NumberFormatter Observability
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+
+## Purpose
+
+Per-package OpenTelemetry instruments for `@suaveplan/i18n-formatter-number`. Exposes a histogram (`i18n.format.number.duration_ms`) recording wall-clock duration of every `formatNumber` call and a counter (`i18n.format.cache_hit`) tagged with `formatter: "number"` that increments on every cache reuse. The two instruments are constructed once via `metrics.getMeter` and are lazily initialised on first record so cold-start latency stays low.
+
+The module also retains an in-memory sample ring (`getNumberFormatMetricsState()` and `resetNumberFormatMetricsState()`) so spec-conformance tests can assert the exact attribute payloads produced by `recordNumberFormat` and `recordNumberCacheHit` without needing to wire a full SDK exporter inside every test file.
+
+## Features
+
+- Lazy instrument construction: the first call to `recordNumberFormat` or `recordNumberCacheHit` builds both instruments via the OTel global meter
+- Stable histogram name `i18n.format.number.duration_ms` with attributes `{ locale, style }`
+- Stable counter name `i18n.format.cache_hit` with attributes `{ formatter: "number" }`
+- In-memory sample ring (`numberDurationSamples`, `cacheHitSamples`) for spec-conformance assertions
+- `resetNumberFormatMetricsState()` clears both arrays so tests stay isolated
+
+## Usage: Basic Example
+
+```typescript
+import {
+    recordNumberFormat,
+    recordNumberCacheHit,
+    getNumberFormatMetricsState,
+    resetNumberFormatMetricsState,
+} from "@suaveplan/i18n-formatter-number/observability";
+
+resetNumberFormatMetricsState();
+recordNumberFormat("en-GB", "decimal", 1.23);
+recordNumberCacheHit();
+const state = getNumberFormatMetricsState();
+state.numberDurationSamples; // [{ attributes: { locale: "en-GB", style: "decimal" } }]
+state.cacheHitSamples;       // [{ attributes: { formatter: "number" } }]
+```
+
+## Usage: Advanced Example — Asserting OTel emission in a test
+
+```typescript
+import { describe, expect, it } from "@suaveplan/testing/runner";
+import { formatNumber, clearNumberFormatterCache } from "@suaveplan/i18n-formatter-number";
+import {
+    getNumberFormatMetricsState,
+    resetNumberFormatMetricsState,
+} from "@suaveplan/i18n-formatter-number/observability";
+
+describe("metrics emission", () => {
+    it("emits duration histogram and cache-hit counter", { timeout: 5000 }, () => {
+        clearNumberFormatterCache();
+        resetNumberFormatMetricsState();
+        formatNumber(1, "en-GB", { style: "decimal" });
+        formatNumber(1, "en-GB", { style: "decimal" });
+        const state = getNumberFormatMetricsState();
+        expect(state.numberDurationSamples).toHaveLength(2);
+        expect(state.cacheHitSamples).toHaveLength(1);
+    });
+});
+```
+
+## API Reference
+
+### `recordNumberFormat(locale: string, style: string, durationMs: number): void`
+
+Records one observation on the `i18n.format.number.duration_ms` histogram and pushes a `{ attributes: { locale, style } }` entry into the in-memory sample ring.
+
+### `recordNumberCacheHit(): void`
+
+Adds 1 to the `i18n.format.cache_hit` counter with `formatter: "number"` and pushes a matching entry into the in-memory sample ring.
+
+### `getNumberFormatMetricsState(): NumberFormatMetricsState`
+
+Returns the in-memory sample ring (live reference — do not mutate). Use `resetNumberFormatMetricsState` between tests to keep state isolated.
+
+### `resetNumberFormatMetricsState(): void`
+
+Empties both sample arrays in place.
+
+### `NumberFormatMetricsState`
+
+`{ numberDurationSamples: FormatMetricSample[]; cacheHitSamples: FormatMetricSample[] }` where `FormatMetricSample = { attributes: Record<string, string | number> }`.
+
+## Implementation Notes
+
+The instruments are built lazily so that consumers who never call `formatNumber` (e.g. tree-shaken builds that import only types) do not pay the OTel meter-lookup cost. The cache check is a strict identity check against the closure-scoped `cached` slot — once built, the instruments are never rebuilt, even across `resetNumberFormatMetricsState` calls (resetting state does not invalidate the meter).
+
+The module deliberately keeps the in-memory sample ring unbounded. Tests should call `resetNumberFormatMetricsState()` between cases. Long-running production processes do not retain the ring because the slim-core wave swaps the in-package observability module for the unified one in `@suaveplan/i18n` core; for now, every leaf package owns its own ring per the design doc's per-package shape.