fix(render): keep authored surface translucency on composite textures

The user reported wielded items subtly hiding particle effects, as if a
translucent texture were missing. Root cause verified in source: the
DAT authors a per-surface Translucency float, and the shared-atlas
extraction honors it by baking (1 - Translucency) into the texture
alpha (MeshExtractor). But a surface with an appearance override -
ObjDesc subpalettes or texture changes, which wielded loot typically
carries - routes through the per-instance composite paths instead
(WbDrawDispatcher.ResolveTexture -> TextureCache
GetOrUploadWithPaletteOverrideBindless /
GetOrUploadWithOrigTextureOverrideBindless -> DecodeFromDats), and the
textured decode there never saw the authored value: only the
Base1Solid branch passed it (SurfaceDecoder.DecodeSolidColor);
DecodeRenderSurface has no translucency input at all.

Consequence: the part still classified translucent, still sorted in
the RetailAlphaQueue, still drew with depth writes off - but with
texture alpha = 1 it overwrote everything already composited behind
it. Particles behind the part vanished; particles in front survived.
The same GfxObj without overrides (atlas path) rendered correctly,
which is why the loss was so selective and subtle.

Fix: SurfaceDecoder.ApplyAuthoredTranslucency mirrors the atlas bake
(in-place alpha scale, caller-owned buffers, Magenta sentinel
guarded), and DecodeFromDats applies it behind an opt-in flag set by
exactly the two world composite paths. The sky path stays unbaked (its
shader applies the authored opacity separately - baking would
double-apply, the AP-89 compounding class) and particle sheets stay
unbaked (emitter-driven alpha, no authored-translucency consumer).
Composite cache keys already include the surface id, so the baked
alpha is cache-coherent.

This closes an unregistered divergence (no register row existed; the
fix restores parity with the shipped atlas mechanism, so none is
added). Investigation evidence: equipped children and world objects
share the same classification chain (ClassifyPackedBatches/GroupKey),
so the gap was override-driven, not attachment-driven - a dropped item
with the same ObjDesc was equally affected.

Core SurfaceDecoder tests 22/22 (3 new); App Release suite 3,968 / 3
skips. Visual gate: a wielded item with authored-translucent parts
must let its particle effects show through.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.Core/Textures/SurfaceDecoder.cs

@@ -93,6 +93,26 @@ public static class SurfaceDecoder
             Height: 1);
     }
 
+    /// <summary>
+    /// Scale a decoded texture's alpha channel by a surface's authored translucency
+    /// (AC's convention: 0.0 fully opaque, 1.0 fully transparent). This is the same
+    /// bake the shared-atlas extraction applies (<c>MeshExtractor</c>,
+    /// <c>alphaScale = 1 - Surface.Translucency</c>); runtime composite decodes must
+    /// apply it too or an override-carrying surface silently loses its authored
+    /// translucency. Scales IN PLACE and returns the same instance — the caller must
+    /// own the buffer (never pass a shared/cached texture such as
+    /// <see cref="DecodedTexture.Magenta"/>).
+    /// </summary>
+    public static DecodedTexture ApplyAuthoredTranslucency(DecodedTexture texture, float translucency)
+    {
+        if (translucency <= 0f) return texture;
+        float alphaScale = Math.Clamp(1f - translucency, 0f, 1f);
+        byte[] rgba = texture.Rgba8;
+        for (int i = 3; i < rgba.Length; i += 4)
+            rgba[i] = (byte)(rgba[i] * alphaScale);
+        return texture;
+    }
+
     /// <summary>
     /// Decode single-byte-per-pixel alpha (PFID_A8 / PFID_CUSTOM_LSCAPE_ALPHA) into RGBA8.
     /// When <paramref name="isAdditive"/> is true: R=G=B=A=val (terrain alpha masks and