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>

tests/AcDream.Core.Tests/Lighting/LightManagerTests.cs

@@ -0,0 +1,139 @@
+using System.Numerics;
+using AcDream.Core.Lighting;
+using Xunit;
+
+namespace AcDream.Core.Tests.Lighting;
+
+public sealed class LightManagerTests
+{
+    private static LightSource MakePoint(Vector3 pos, float range, uint ownerId = 0, bool lit = true)
+        => new LightSource
+        {
+            Kind = LightKind.Point,
+            WorldPosition = pos,
+            Range = range,
+            IsLit = lit,
+            OwnerId = ownerId,
+        };
+
+    [Fact]
+    public void Register_Unregister_TracksList()
+    {
+        var mgr = new LightManager();
+        var a = MakePoint(Vector3.Zero, 5f);
+        var b = MakePoint(new Vector3(10, 0, 0), 5f);
+        mgr.Register(a);
+        mgr.Register(b);
+        Assert.Equal(2, mgr.RegisteredCount);
+
+        mgr.Unregister(a);
+        Assert.Equal(1, mgr.RegisteredCount);
+    }
+
+    [Fact]
+    public void Register_DuplicateInstance_Idempotent()
+    {
+        var mgr = new LightManager();
+        var light = MakePoint(Vector3.Zero, 5f);
+        mgr.Register(light);
+        mgr.Register(light);
+        Assert.Equal(1, mgr.RegisteredCount);
+    }
+
+    [Fact]
+    public void Tick_SelectsByDistance_Top8()
+    {
+        var mgr = new LightManager();
+        // 12 lights at varying distances, all with range 100 so none filter out.
+        for (int i = 0; i < 12; i++)
+            mgr.Register(MakePoint(new Vector3(i, 0, 0), 100f));
+
+        mgr.Tick(viewerWorldPos: Vector3.Zero);
+
+        Assert.Equal(8, mgr.ActiveCount);
+        // Top 8 should be the closest (i=0..7).
+        foreach (var l in mgr.Active)
+        {
+            Assert.NotNull(l);
+            Assert.True(l!.WorldPosition.X <= 7f);
+        }
+    }
+
+    [Fact]
+    public void Tick_DropsLightsOutsideRangeWithSlack()
+    {
+        var mgr = new LightManager();
+        mgr.Register(MakePoint(new Vector3(20, 0, 0), range: 5f));  // far outside its own range
+
+        mgr.Tick(viewerWorldPos: Vector3.Zero);
+
+        Assert.Equal(0, mgr.ActiveCount);
+    }
+
+    [Fact]
+    public void Tick_IncludesLightsNearRangeEdge_WithSlack()
+    {
+        var mgr = new LightManager();
+        // Light at distance 5.0, range 5.0: distSq=25, rangeSq*1.1^2 = 25*1.21 = 30.25 → included.
+        mgr.Register(MakePoint(new Vector3(5, 0, 0), range: 5f));
+
+        mgr.Tick(viewerWorldPos: Vector3.Zero);
+        Assert.Equal(1, mgr.ActiveCount);
+    }
+
+    [Fact]
+    public void Tick_SunSlot0_PreservedAcrossTicks()
+    {
+        var mgr = new LightManager();
+        var sun = new LightSource { Kind = LightKind.Directional, WorldForward = -Vector3.UnitZ };
+        mgr.Sun = sun;
+
+        mgr.Register(MakePoint(Vector3.Zero, 100f));
+        mgr.Tick(Vector3.Zero);
+
+        Assert.Equal(2, mgr.ActiveCount);
+        Assert.Same(sun, mgr.Active[0]);
+    }
+
+    [Fact]
+    public void Tick_UnlitLight_Excluded()
+    {
+        var mgr = new LightManager();
+        var light = MakePoint(Vector3.Zero, 100f, lit: false);
+        mgr.Register(light);
+
+        mgr.Tick(Vector3.Zero);
+        Assert.Equal(0, mgr.ActiveCount);
+
+        // Toggle lit: should now appear.
+        light.IsLit = true;
+        mgr.Tick(Vector3.Zero);
+        Assert.Equal(1, mgr.ActiveCount);
+    }
+
+    [Fact]
+    public void UnregisterByOwner_RemovesAttachedLights()
+    {
+        var mgr = new LightManager();
+        mgr.Register(MakePoint(Vector3.Zero, 5f, ownerId: 42));
+        mgr.Register(MakePoint(new Vector3(1, 0, 0), 5f, ownerId: 42));
+        mgr.Register(MakePoint(new Vector3(2, 0, 0), 5f, ownerId: 99));
+
+        mgr.UnregisterByOwner(42);
+        Assert.Equal(1, mgr.RegisteredCount);
+    }
+
+    [Fact]
+    public void DistSq_UpdatedEachTick()
+    {
+        var mgr = new LightManager();
+        var light = MakePoint(new Vector3(3, 0, 4), 10f);   // dist 5
+        mgr.Register(light);
+
+        mgr.Tick(Vector3.Zero);
+        Assert.Equal(25f, light.DistSq, 2);
+
+        mgr.Tick(new Vector3(3, 0, 0));   // same x, same y, z diff 4
+        Assert.Equal(16f, light.DistSq, 2);
+    }
+}