Simplify asset loader state and shutdown

Asset requests carried duplicate phase, generation, and notification state.
Remove unused texture dependencies and project-path storage, derive conversion
from owned mesh data, and use request identity, cancellation, and the stopping
flag to reject obsolete results.

Decode cached meshes in the library's worker build service so serialized bytes
stay local and completions retain only decoded data and a cache-origin flag.
Make draining private to the loader destructor and remove duplicate shutdown
calls from library unload and destruction. Document the resulting lifecycle.

Validation: macOS Qt 6.10 Debug build; 14 first/repeat CLI exports; temporary
lifecycle checks covering cold/warm/corrupt caches, notification delivery and
suppression, pause/resume, cancellation, destructor-only draining, and reload.

docs/asset-loading.md

@@ -233,6 +233,9 @@ The diagram shows the normal paths. Mesh parsing and conversion are separate
 worker phases, so a part can return to `Queued` between phases. Invalidation or
 unloading resets readiness; cancellation and stale-result disposal also have
 request-level terminal flags, independent of the published asset state.
+Queued piece and synth requests use the presence of `MeshData` to select
+conversion; an empty pointer selects parsing or generation. `Running` tracks
+outstanding worker phases through completion processing.
 
 A part becomes `Loaded` after its private mesh is committed and all declared
 textures are ready. Valid empty geometry may also settle successfully. A missing
@@ -306,11 +309,12 @@ 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 on workers; 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 as successful mesh
-caches. Temporary disk textures are scoped by their resolved file identity, so
-same-name PNGs in different project directories remain distinct.
+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
+as successful mesh caches. Temporary disk textures are scoped by their resolved
+file identity, so same-name PNGs in different project directories remain distinct.
 
 Mesh cache validity includes the cache format, source checksums, stud style, and
 stud-cylinder color setting. Style flags propagate through nested primitive
@@ -539,8 +543,9 @@ Published ordinary meshes are rebuilt selectively using `HasStyleStud`, includin
 flags propagated from nested includes; a style change does not indiscriminately
 invalidate every library mesh. Pending snapshots must still be refreshed so
 obsolete settings cannot publish later.
-Library reload and destruction call `CancelAndDrain()` before destroying sources,
-registries, or GL resources. It advances the generation, cancels pending work,
+Library reload and destruction destroy the loader before releasing sources,
+registries, or GL resources. The loader destructor calls its private
+`CancelAndDrain()`, which stops dispatch, cancels pending work,
 joins workers, discards obsolete results, and releases notification holds without
 requiring successful upload or delivering public settlement signals.
 
@@ -591,7 +596,7 @@ suite.
   GL upload, or settlement callbacks. Condition waits release the queue mutex
   while sleeping. Do not hold library locks needed by workers while draining.
 - Preserve in-flight holds through stale-result disposal and deferred notification.
-  Validate request identity/generation before publishing; deleting an owner must
+  Validate request identity and cancellation before publishing; deleting an owner must
   not let a worker or callback access that deleted model/piece.
 - Preserve unresolved dependency names and diagnostics through caches. A failed
   include/texture must never be treated as successful partial asset loading.