refactor(pipeline): MP1a cleanup - narrow public surface, csproj parity, move residue

Coordinator-directed final cleanup before the user gate; none behavioral:

1. MeshExtractor public surface narrowed to the cross-assembly entry
   points App actually calls (PrepareMeshData, PrepareCellStructMeshData,
   CollectParts, ComputeBounds); PrepareSetupMeshData,
   CollectEmittersFromScript, PrepareGfxObjMeshData,
   PrepareEnvCellMeshData, PrepareCellStructEdgeLineData back to private
   (internal dispatch, only reached via PrepareMeshData).
2. sideStagedSink constructor parameter is now REQUIRED (no default;
   type stays nullable for a conscious null): a bake tool that forgot
   the sink would silently lose particle-preload meshes.
3. AcDream.Content.csproj gains TreatWarningsAsErrors + LangVersion
   latest (parity with AcDream.Core.csproj). Surfaced zero warnings.
4. Dead usings removed from ObjectMeshManager.cs (BCnEncoder.*,
   SixLabors.*) — the inline decode moved out in Task 4.
5. Doc fixes: ObjectMeshData.cs cross-assembly <see cref> ->
   plain text (Content can't resolve App types); IDatReaderWriter.cs
   stale Phase O-T7 'both in this namespace' sentence rewritten.
6. Stale test doc comments updated to MeshExtractor.PrepareGfxObjMeshData
   (StipplingSurfaceEquivalenceTests, Issue119UpNullGfxObjDumpTests) —
   comments only, no code/assertion changes.

dotnet build green (0 warnings in Content under warnings-as-errors);
full test suite 4059 passed / 0 failed / 4 skipped.

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

src/AcDream.Content/AcDream.Content.csproj

@@ -4,6 +4,8 @@
     <TargetFramework>net10.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>