feat(lighting): Phase G.2 LightSource + LightManager (data + selection)

Retail-faithful 8-light cap selection (r13 §12) — the fixed-function
D3D pipeline's "hardware lights" constraint carried over to modern GL
via UBO-per-draw.

Core layer (AcDream.Core/Lighting):
- LightSource: Kind (Directional/Point/Spot), WorldPosition,
  WorldForward, ColorLinear, Intensity, Range (hard cutoff),
  ConeAngle (spot), OwnerId (entity attachment), IsLit latch.
- CellAmbientState: (AmbientColor, SunColor, SunDirection) sourced from
  R12 sky state for outdoor cells or EnvCell dat for indoor cells.
- LightManager: Register/Unregister/UnregisterByOwner/Clear + Tick
  per frame. Selection matches r13 §12.2 exactly:
  1) Skip unlit + directional.
  2) Compute DistSq for every registered point/spot.
  3) Drop lights outside Range² * 1.1 (10% slack prevents pop).
  4) Sort by DistSq ascending; take up to 7 (slot 0 reserved for Sun).
  5) Slot 0 = Sun (Directional); slots 1..7 = nearest in-range.

Tests (9 new):
- Register/Unregister/Idempotent register.
- Tick picks top 8 by distance when 12 registered.
- Range filter drops far lights (5.0 range, 20m away).
- Range slack includes lights at exactly the boundary.
- Sun reserved at slot 0 across ticks.
- Unlit lights excluded; toggling IsLit brings them back.
- UnregisterByOwner removes all owner's lights.
- DistSq updated each tick for viewer movement.

Build green, 596 tests pass (up from 587).

Next: wire LightManager into the shader UBO pass (G.2 second commit)
and feed Sun from WorldTimeService.CurrentSunDirection per frame.

Ref: r13 §10.2 (D3D attenuation = none inside Range + hard cutoff),
§12 (full port plan).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Lighting/LightSource.cs

@@ -0,0 +1,63 @@
+using System;
+using System.Numerics;
+
+namespace AcDream.Core.Lighting;
+
+/// <summary>
+/// Retail light-source kinds (r13 §12.1).
+/// </summary>
+public enum LightKind
+{
+    Directional = 0,  // sun, moon — no position, infinite range
+    Point       = 1,  // torch, fireplace, spell aura
+    Spot        = 2,  // cone-shaped (rare in AC, used for a few specific lamps)
+}
+
+/// <summary>
+/// Per-frame light record. Used by <see cref="LightManager"/> and fed to
+/// the shader UBO on every draw call.
+///
+/// <para>
+/// Retail semantics (r13 §10.2):
+/// <list type="bullet">
+///   <item><description>
+///     Hard cutoff at <see cref="Range"/> — no smoothstep, no distance
+///     attenuation inside the range. "Crisp bubble of illumination."
+///   </description></item>
+///   <item><description>
+///     Max 8 active lights (<see cref="LightManager.MaxActiveLights"/>),
+///     ranked by distance-to-viewer. Slot 0 is reserved for the sun.
+///   </description></item>
+///   <item><description>
+///     <see cref="IsLit"/> latches the <c>SetLightHook</c> value so
+///     animations can toggle a light on/off (torch being lit, light
+///     crystal activating).
+///   </description></item>
+/// </list>
+/// </para>
+/// </summary>
+public sealed class LightSource
+{
+    public LightKind Kind;
+    public Vector3   WorldPosition;
+    public Vector3   WorldForward;    // for Spot/Directional
+    public Vector3   ColorLinear = Vector3.One;   // R,G,B in [0,1], pre-brightness
+    public float     Intensity = 1f;
+    public float     Range = 10f;     // metres, hard cutoff
+    public float     ConeAngle = 0f;  // radians, Spot only
+    public uint      OwnerId;         // attached entity id; 0 = world-global
+    public bool      IsLit = true;    // SetLightHook latch
+
+    // Cached each frame by LightManager.
+    public float DistSq;
+}
+
+/// <summary>
+/// Per-cell ambient + sun state. For outdoor cells this comes from the
+/// R12 sky state; for indoor cells the EnvCell dat carries a per-cell
+/// ambient override (r13 §3).
+/// </summary>
+public readonly record struct CellAmbientState(
+    Vector3 AmbientColor,
+    Vector3 SunColor,
+    Vector3 SunDirection);