fix(lighting): match retail indoor ambient (0.20 neutral, not 0.10/0.09/0.08 warm)

Indoor cells rendered "almost black" because the hardcoded ambient at
GameWindow.cs:8342-8345 was an early-2026 guess (0.10, 0.09, 0.08 — half
retail brightness, warm-tinted) rather than the retail value. The named
retail decomp (acclient.pdb, Sept 2013 EoR build) shows
CellManager::ChangePosition @ 0x004559B0 calls
SmartBox::SetWorldAmbientLight(0.2f, 0xFFFFFFFF) whenever the player's
CObjCell::seen_outside flag is 0 — a flat 0.20 white floor, not a
dungeon-tone warm color.

Investigation also confirmed:
- EnvCell.dat does NOT carry inline lights — CEnvCell::UnPack reads
  numVisibleCells where Binary Ninja's heuristic decomp inferred
  "num_lights". Retail's CObjCell.light_list is populated at runtime via
  add_light() calls from neighbouring cell light registrations + per-cell
  static-object Setup.Lights, NOT from the dat byte stream.
- Setup.Lights from indoor static objects (entity.SourceGfxObjOrSetupId
  prefix 0x02xxxxxx) DO flow through LightInfoLoader.Load (line 5765)
  and reach LightManager via LightingHookSink. The wire is intact; the
  per-frame Tick + UBO upload chain (line 6865-6867) is intact.
- Retail's particle system does NOT emit lights from particles themselves.
  The light comes from the owning Setup's LightInfo records.

Pre-existing failures in DispatcherToMovementIntegrationTests, BSPStepUpTests,
and MotionInterpreterTests are on the branch already and unrelated to this
change (verified by stashing + retesting).

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -8319,7 +8319,13 @@ public sealed class GameWindow : IDisposable
     /// Derive the current sun (directional light, slot 0 of the UBO)
     /// from the interpolated <see cref="AcDream.Core.World.SkyKeyframe"/>,
     /// plus the cell ambient. Indoor cells force the sun intensity to
-    /// zero (r13 §13.7) and substitute a fixed dungeon-tone ambient.
+    /// zero and substitute a flat 0.2 white ambient — exact retail
+    /// behavior per <c>CellManager::ChangePosition</c> @ 0x004559B0,
+    /// which calls <c>SmartBox::SetWorldAmbientLight(0.2f, 0xFFFFFFFF)</c>
+    /// when the player's <c>CObjCell::seen_outside</c> flag is 0.
+    /// Indoor brightness then comes from per-cell point lights
+    /// (Setup.Lights on the cell's static objects, registered through
+    /// <see cref="AcDream.Core.Lighting.LightingHookSink"/>).
     /// </summary>
     private void UpdateSunFromSky(AcDream.Core.World.SkyKeyframe kf, bool cameraInsideCell)
     {
@@ -8330,7 +8336,8 @@ public sealed class GameWindow : IDisposable
 
         if (cameraInsideCell)
         {
-            // Dungeon default per r13 §3 — warm-dark ambient, no sun.
+            // Indoor default — retail's flat 0.2 neutral ambient, sun
+            // zeroed. See xref to retail decomp in the doc comment above.
             Lighting.Sun = new AcDream.Core.Lighting.LightSource
             {
                 Kind = AcDream.Core.Lighting.LightKind.Directional,
@@ -8340,7 +8347,7 @@ public sealed class GameWindow : IDisposable
                 Range = 1f,
             };
             Lighting.CurrentAmbient = new AcDream.Core.Lighting.CellAmbientState(
-                AmbientColor: new System.Numerics.Vector3(0.10f, 0.09f, 0.08f),
+                AmbientColor: new System.Numerics.Vector3(0.20f, 0.20f, 0.20f),
                 SunColor:     System.Numerics.Vector3.Zero,
                 SunDirection: sunToWorld);
         }