Unify mesh color resolution and section finalization

Cached meshes retained color-dependent flags and section ordering from the palette used when they were saved. Finalize fresh piece, synth, and cached mesh sections on the UI thread so color indices, flags, and ordering all reflect the current palette before publication.

Keep color codes in private mesh sections until finalization and remove separate color-code vectors, worker palette snapshots, and the early color-resolution path. Geometry conversion now runs without global color access or texture lookup callbacks.

docs/asset-loading.md

@@ -283,7 +283,7 @@ worker scheduling and `RunWorker()`, UI-thread `ProcessCompletions()` dispatch t
 `ProcessPieceCompletion()`, `ProcessSynthCompletion()`, or `ProcessTextureCompletion()`, dependency settlement
 and publication, then deferred notifications. Readiness waits follow these paths;
 invalidation, pause/resume, and shutdown are grouped at the end of the file.
-Piece and synth handlers share color preparation and texture dependency checks,
+Piece and synth handlers share mesh color finalization and texture dependency checks,
 while keeping their distinct ownership and publication rules explicit.
 
 ```mermaid
@@ -297,9 +297,9 @@ sequenceDiagram
     Library->>Loader: Queue/promote
     Loader->>Worker: Immutable source snapshot
     Worker-->>Loader: Private mesh data or cached mesh
-    Loader->>Loader: Resolve global colors
     Loader->>Worker: Convert source mesh data
     Worker-->>Loader: Private mesh
+    Loader->>Loader: Set section colors, update flags, and sort sections
     Loader->>Loader: Resolve textures and queue decode if needed
     Loader->>Worker: Texture source snapshot
     Worker-->>Loader: Decoded CPU image
@@ -310,9 +310,13 @@ sequenceDiagram
 Texture-free meshes skip decoding/upload; cached meshes skip source parsing and
 conversion. Both paths use the same dependency and publication rules. Cache
 reads and mesh deserialization run in the library's worker build service; cache
-bytes stay local, and the result records whether it came from cache. Source mesh
-color indices are resolved on the UI thread before conversion uses a color
-snapshot. A corrupt mesh cache is rebuilt privately. Failed builds are not saved
+bytes stay local, and the result records whether it came from cache. Private mesh
+sections store color codes in `ColorIndex` until finalization converts them to
+indices. Fresh pieces, synths, and cached meshes all use
+`FinalizeSections()` on the UI thread to resolve color indices, refresh
+solid/translucent/default flags, and sort sections using the current palette.
+Workers build geometry without reading global colors or keeping palette snapshots.
+A corrupt mesh cache is rebuilt privately. Failed builds are not saved
 as successful mesh caches. Temporary disk textures are scoped by their resolved
 file identity, so same-name PNGs in different project directories remain distinct.