feat(net+app): SubPalette overlays applied to palette-indexed textures (Phase 5b)

Implements the other half of ObjDesc: SubPalettes (palette-range
overlays) that repaint palette-indexed textures with per-entity color
schemes. Ported algorithm from ACViewer Render/TextureCache.IndexToColor
after the user pointed out I was prematurely implementing from scratch
instead of checking all the reference repos.

The Nullified Statue of a Drudge sends (setup=0x020007DD with a drudge
GfxObj animPart replacing part 1, plus 2 texChanges targeted at part 1,
plus 1 subpalette id=0x04001351 offset=0 length=0). The TextureChanges
swap fine detail surfaces; the SubPalette with length=0 ("entire palette"
per Chorizite docs) remaps the drudge's flesh-tone palette to stone.
Without this commit, the statue looked like a normal flesh drudge
because palette-indexed textures decoded with the base flesh palette.

Added:
  - Core/World/PaletteOverride.cs: per-entity record carrying
    BasePaletteId + a list of (SubPaletteId, Offset, Length) range
    overlays. Documents the "offset/length are wire-scaled by 8"
    convention and the "length=0 means whole palette" sentinel.
  - WorldEntity.PaletteOverride nullable field. Per-entity (same across
    all parts), in contrast to MeshRef.SurfaceOverrides which is per-part.
  - TextureCache.GetOrUploadWithPaletteOverride: new entry point that
    composes the effective palette at decode time. Composite cache key
    is (surfaceId, origTexOverride, paletteHash) so entities with
    equivalent palette setups share the GL texture.
  - ComposePalette: ports ACViewer's IndexToColor overlay loop:
      for each subpalette sp:
          startIdx = sp.Offset * 8             // multiply back from wire
          count = sp.Length == 0 ? 2048 : sp.Length * 8   // sentinel
          for j in [0, count):
              composed[j + startIdx] = subPal.Colors[j + startIdx]
    Critical detail: copies from the SAME offset in the sub palette, not
    from [0]. Both base and sub are treated as full palettes sharing an
    index space.
  - StaticMeshRenderer.Draw: three-way switch on (entity.PaletteOverride,
    meshRef.SurfaceOverrides) picks the right TextureCache path:
      - Both → palette override (it handles origTex override internally)
      - Only tex override → GetOrUploadWithOrigTextureOverride
      - Neither → plain GetOrUpload
  - GameWindow.OnLiveEntitySpawned: builds PaletteOverride from
    spawn.BasePaletteId + spawn.SubPalettes when the server sent any.

Reference note: the user asked "but I mean THIS MUST BE IN WORLDBUILDER"
which was the right push. WorldBuilder is actually a dat VIEWER and its
ClothingTableBrowserViewModel is a 10-line stub — it doesn't apply
palette overlays because it doesn't need to. The actual algorithm lives
in ACViewer (a MonoGame character viewer), which I should have checked
earlier. CLAUDE.md updated with a standing rule: always cross-reference
all four of references/ACE, ACViewer, WorldBuilder, Chorizite.ACProtocol,
plus holtburger. A single reference can be misleading; the intersection
is usually the truth.

Tests: 77 core + 83 net = 160, all green.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -710,6 +710,23 @@ public sealed class GameWindow : IDisposable
             return;
         }
 
+        // Build optional per-entity palette override from the server's base
+        // palette + subpalette overlays. The renderer applies these to
+        // palette-indexed textures (PFID_P8 / PFID_INDEX16) to get per-entity
+        // skin/hair/body colors and statue stone recoloring. Non-palette
+        // textures ignore the override.
+        AcDream.Core.World.PaletteOverride? paletteOverride = null;
+        if (spawn.SubPalettes is { Count: > 0 } spList)
+        {
+            var ranges = new AcDream.Core.World.PaletteOverride.SubPaletteRange[spList.Count];
+            for (int i = 0; i < spList.Count; i++)
+                ranges[i] = new AcDream.Core.World.PaletteOverride.SubPaletteRange(
+                    spList[i].SubPaletteId, spList[i].Offset, spList[i].Length);
+            paletteOverride = new AcDream.Core.World.PaletteOverride(
+                BasePaletteId: spawn.BasePaletteId ?? 0,
+                SubPalettes: ranges);
+        }
+
         var entity = new AcDream.Core.World.WorldEntity
         {
             Id = _liveEntityIdCounter++,
@@ -717,6 +734,7 @@ public sealed class GameWindow : IDisposable
             Position = worldPos,
             Rotation = rot,
             MeshRefs = meshRefs,
+            PaletteOverride = paletteOverride,
         };
 
         var snapshot = new AcDream.Plugin.Abstractions.WorldEntitySnapshot(