fix(pipeline): MP1a - sink delegate restores immediate side-stage enqueue (exception-path faithfulness)

Coordinator-directed follow-up. The buffer-and-drain seam diverged from
the original on the exception path: pre-MP1a, CollectEmittersFromScript
enqueued particle-preload meshes DIRECTLY into _stagedMeshData
mid-Prepare, so preloads staged before a later throw in the same
Prepare* call (reachable via PrepareEnvCellMeshData side-staging during
its StaticObjects loop, then PrepareCellStructMeshData throwing on a
malformed-dat texture decode) were already safely enqueued. The drain
version only flushed after a successful return — on throw, entries
stranded on the shared extractor until an unrelated successful call
flushed them, and were silently dropped on dispose.

Fix: MeshExtractor takes an Action<ObjectMeshData>? sideStagedSink
constructor parameter; the two CollectEmittersFromScript sites become
_sideStagedSink?.Invoke(meshData) — the original code shape (immediate
hand-off) at those exact lines. ObjectMeshManager wires the sink to
_stagedMeshData.Enqueue, restoring the original immediate-enqueue
semantics including on mid-Prepare throw. _sideStaged buffer,
DrainSideStaged(), and the ProcessQueueAsync drain loop are deleted.
The MP1b bake tool passes its own collector.

Inventory doc updated: MP1a note now records the sink seam and the
Content-owned upload enums, so its no-behavior-change claim is accurate.

dotnet build green; full test suite 4059 passed / 0 failed / 4 skipped.

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

docs/architecture/worldbuilder-inventory.md

@@ -53,8 +53,26 @@ behavior change, no divergence-register row.
 - `src/AcDream.Content/TextureKey.cs` — the atlas dedup key, lifted out of
   the GL-owning `TextureAtlasManager` (which stays in App and now
   references the lifted struct).
+- `src/AcDream.Content/UploadFormats.cs` — Content-owned
+  `UploadPixelFormat`/`UploadPixelType` enums carried by
+  `MeshBatchData`/`TextureBatchData` instead of
+  `Silk.NET.OpenGL.PixelFormat`/`PixelType` (Content must stay
+  Silk.NET-free — the bake tool must not ship GL binaries). Underlying
+  values are the GL ABI constants, numerically identical to the Silk.NET
+  members; App casts at its single upload boundary (the `AddTexture` call
+  in `UploadGfxObjMeshData`) via a lifted nullable enum conversion —
+  value- and null-preserving.
 - `src/AcDream.Content/IDatReaderWriter.cs`, `EdgeLineBuilder.cs` — GL-free
   dependencies of the extractor, moved (namespace-only) alongside it.
+- **Side-stage sink seam:** `CollectEmittersFromScript` pre-loads particle
+  GfxObj meshes mid-extraction and, pre-MP1a, enqueued them directly onto
+  `ObjectMeshManager._stagedMeshData`. The extractor now takes an
+  `Action<ObjectMeshData>? sideStagedSink` constructor parameter; App wires
+  it to `_stagedMeshData.Enqueue`, preserving the original
+  immediate-enqueue semantics exactly — including on a mid-`Prepare*`
+  throw (preloads staged before a malformed-dat texture-decode exception
+  survive, as they always did). The MP1b bake tool passes its own
+  collector.
 - **Stays in `src/AcDream.App/Rendering/Wb/`:** `ObjectMeshManager` (now a
   thin wrapper owning the staged-queue/worker-pool/Dispose-quiesce lifecycle
   and all GL upload — constructs one `MeshExtractor` and delegates every