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+# slider-helpers (module-private)
+
+## Purpose
+
+Shared pure functions for both `<SliderV>` and `<SliderH>` so that
+value resolution, detent snapping, and keyboard stepping are identical
+across the two orientations. The helpers live under `slider-v/` for a
+stable physical home, and `<SliderH>` reaches across with a single
+relative import. They are not re-exported from the package barrel —
+consumers of the package only see `<SliderV>` and `<SliderH>`.
+
+## Features
+
+- `clamp` / `quantize` — primitive numeric helpers.
+- `applyDetents` — within-threshold snap to the nearest detent `at`.
+- `resolveSliderValue` — full pipeline: clamp → step quantize →
+  detent snap.
+- `fractionForValue` — normalize value to 0..1 for thumb-position math.
+- `keyboardStep` — returns the next value for arrow / Home / End /
+  PageUp / PageDown keys, orientation-aware, Shift-fine-mode aware.
+  Returns `null` for unrecognised keys.
+
+## Usage: Basic
+
+```ts
+import {
+    resolveSliderValue,
+    fractionForValue,
+} from "./slider-helpers.js";
+
+const next = resolveSliderValue({
+    raw: 0.42,
+    min: 0,
+    max: 1,
+    step: 0.05,
+});
+const pct = fractionForValue(next, 0, 1); // 0.4
+```
+
+## Usage: Advanced
+
+Keyboard-driven step with custom fine factor and detents:
+
+```ts
+import {
+    keyboardStep,
+    resolveSliderValue,
+} from "./slider-helpers.js";
+
+const proposed = keyboardStep({
+    key: event.key,
+    value: current,
+    min: -60,
+    max: 12,
+    step: 0.5,
+    fineFactor: 0.1,
+    shiftKey: event.shiftKey,
+    orientation: "vertical",
+});
+if (proposed !== null) {
+    const snapped = resolveSliderValue({
+        raw: proposed,
+        min: -60,
+        max: 12,
+        step: 0.5,
+        detents: [{ at: 0, label: "0 dB" }],
+        snapThreshold: 0.5,
+    });
+    onChange(snapped);
+}
+```
+
+## API Reference
+
+| Function             | Signature                                                                      |
+| -------------------- | ------------------------------------------------------------------------------ |
+| `clamp`              | `(value, min, max) => number`                                                  |
+| `quantize`           | `(value, step, min) => number`                                                 |
+| `applyDetents`       | `(value, detents, threshold) => number`                                        |
+| `resolveSliderValue` | `({ raw, min, max, step, detents?, snapThreshold? }) => number`                |
+| `fractionForValue`   | `(value, min, max) => number` (always clamped to 0..1)                         |
+| `keyboardStep`       | `(args) => number \| null`; returns `null` for unrecognised keys.              |
+
+## Implementation Notes
+
+`applyDetents` greedily picks the closest detent within `threshold`
+distance; `threshold` defaults to 2% of the span when not supplied.
+`quantize` short-circuits when `step <= 0` so consumers can opt out
+of stepping with `step: 0`. `keyboardStep` derives a perceptible
+per-key delta of `max(step, span / 100)` so wide ranges still respond
+to a single arrow key even when `step` is tiny. The orientation flip
+between vertical / horizontal is encoded purely in the key mapping;
+all downstream math is direction-agnostic.