## 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