fix(textures): palette-indexed surfaces + alpha cutout shader

Addresses the 'doors, windows, and alpha-keyed parts render bright
pink' issue the user observed after the Phase 2a visual checkpoint.

SurfaceDecoder gains a second overload taking an optional Palette
parameter. When the render surface format is PFID_INDEX16 and a
palette is supplied, each 16-bit value in SourceData is treated as
an index into Palette.Colors (a List<ColorARGB>) and the corresponding
ARGB color's channels are written to the output buffer. The original
no-palette overload is preserved so the Task 3 unit tests that
confirm INDEX16 -> magenta fallback still describe their behavior
correctly (INDEX16 without a palette still returns magenta).

TextureCache now resolves the RenderSurface's DefaultPaletteId via
the dats and passes the resulting Palette (or null) to the decoder.

mesh.frag adds an alpha cutout: fragments with sampled alpha < 0.5
are discarded. Without this, transparent regions of alpha-keyed
textures (doors, windows, foliage cutouts) would render as opaque
rectangles using the texture's background color. This is the
standard alpha-tested approach, simpler than full alpha blending
and matches how AC's original client rendered these surfaces.

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

src/AcDream.App/Rendering/TextureCache.cs

@@ -49,7 +49,14 @@ public sealed unsafe class TextureCache : IDisposable
         if (rs is null)
             return DecodedTexture.Magenta;
 
-        return SurfaceDecoder.DecodeRenderSurface(rs);
+        // Palette lookup for indexed formats (doors, windows, alpha-keyed foliage).
+        // If DefaultPaletteId is 0 or unresolvable, SurfaceDecoder falls back to magenta
+        // for PFID_INDEX16 surfaces.
+        Palette? palette = rs.DefaultPaletteId != 0
+            ? _dats.Get<Palette>(rs.DefaultPaletteId)
+            : null;
+
+        return SurfaceDecoder.DecodeRenderSurface(rs, palette);
     }
 
     private uint UploadRgba8(DecodedTexture decoded)