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openspec/changes/enterprise-gaming-universe/specs/gaming-environment-lighting/spec.md

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+## ADDED Requirements
+
+### Requirement: Sky colour via Rayleigh/Mie scattering
+
+The system SHALL compute sky colour from sun altitude, `AtmosphereConfig.rayleighCoefficient`, and `mieCoefficient`.
+
+#### Scenario: Earth noon sky
+
+- **WHEN** the sun is at zenith on an Earth-like planet
+- **THEN** the computed `skyColour` is deep blue (high B, moderate G, low R)
+
+#### Scenario: Earth sunset sky
+
+- **WHEN** the sun is near the horizon on an Earth-like planet
+- **THEN** the computed `skyColour` is orange/red (high R, moderate G, low B)
+
+#### Scenario: Mars noon sky
+
+- **WHEN** the sun is at zenith on a Mars-like planet with CO₂ atmosphere
+- **THEN** the computed `skyColour` is butterscotch (high R, moderate G, low B)
+
+#### Scenario: Vacuum world
+
+- **WHEN** the observer's planet has no `AtmosphereConfig`
+- **THEN** `skyColour` is black always
+
+### Requirement: Twilight star visibility thresholds
+
+The system SHALL apply magnitude limits based on sun altitude and twilight phase.
+
+#### Scenario: Civil twilight limit
+
+- **WHEN** sun altitude is between `0` and `-6` degrees
+- **THEN** only stars with `apparentMagnitude <= 1` are visible
+
+#### Scenario: Astronomical twilight limit
+
+- **WHEN** sun altitude is between `-12` and `-18` degrees
+- **THEN** stars with `apparentMagnitude <= 6` are visible
+
+#### Scenario: Full dark
+
+- **WHEN** sun altitude is below `-18` degrees
+- **THEN** all naked-eye stars are visible
+
+#### Scenario: Vacuum collapse
+
+- **WHEN** the planet has no atmosphere
+- **THEN** all stars are visible regardless of sun altitude
+
+### Requirement: Multi-satellite moonlight
+
+The system SHALL compute per-satellite moonlight proportional to phase illumination, angular diameter squared, and albedo, summing across all satellites.
+
+#### Scenario: Single moon night
+
+- **WHEN** a planet with one satellite has a full-moon phase
+- **THEN** `moonContributions` contains one entry with significant light value
+
+#### Scenario: Multiple moons
+
+- **WHEN** a planet with two satellites both above the horizon is evaluated
+- **THEN** `totalAmbientLight` includes the sum of both contributions
+
+### Requirement: Gameplay modifiers
+
+The system SHALL export `EnvironmentModifiers` with `totalAmbientLight`, `stealthBonus`, `visibilityRangeMetres`, and `npcSleepProbability` as continuous functions of ambient light.
+
+#### Scenario: Stealth at midnight
+
+- **WHEN** `totalAmbientLight` is near `0`
+- **THEN** `stealthBonus` is near `1`
+
+#### Scenario: Visibility at noon
+
+- **WHEN** `totalAmbientLight` is near `1`
+- **THEN** `visibilityRangeMetres` is at its maximum
+
+### Requirement: Plain ECS component interfaces
+
+The system SHALL export `DayNightClock`, `LightState`, and `DayNightAffected` as plain TypeScript interfaces with no `@suaveplan/ecs` dependency.
+
+#### Scenario: Interface shapes
+
+- **WHEN** the exports are inspected
+- **THEN** they are `interface` declarations, not class constructors or archetype registrations
+
+### Requirement: RenderPayload shape
+
+The system SHALL export `RenderPayload` as the canonical per-tick output for downstream render integrations.
+
+#### Scenario: Payload completeness
+
+- **WHEN** a render payload is generated at a tick
+- **THEN** it contains `skyColour`, `sunAltitude`, `sunAzimuth`, `horizonColour`, `totalAmbientLight`, `moonContributions`, `fogDensity`, `rayleighTint`, `textures`, and optional `weather`
+
+### Requirement: Texture manifest pass-through
+
+The system SHALL pass `PlanetTextureManifest` identifiers through verbatim without loading or storing assets.
+
+#### Scenario: Texture identifiers
+
+- **WHEN** a `PlanetConfig` with `textures: { diffuse: "earth-diffuse", night: "earth-night" }` is used
+- **THEN** the `RenderPayload.textures` contains the same identifiers
+- **AND** no file loading occurs
+
+### Requirement: Weather state pass-through
+
+The system SHALL include `WeatherState` in `RenderPayload` and `LightState` when a `WeatherProvider` is registered, without breaking when absent.
+
+#### Scenario: No weather provider
+
+- **WHEN** no `WeatherProvider` is registered in the underlying `UniverseClient`
+- **THEN** `RenderPayload.weather` is undefined
+- **AND** lighting computation continues without weather modulation