fix(pipeline): MP1b review - bake CLI determinism + bounded memory

Adversarially-verified review findings 1 and 10:

(1) The bake pipeline no longer accumulates every decoded ObjectMeshData
in one ConcurrentBag before writing (multi-GB OOM risk on the full
bake), and no longer writes blobs in thread-completion order (which
violated the plan's "bakes must be byte-reproducible run-to-run").
New shape in BakeRunner: build the FULL id list, sort by PakKey, chunk
into 512-id batches; Parallel.ForEach WITHIN each batch; sort each
batch's results by key and AddBlob sequentially; release the batch.
Batches are contiguous key ranges, so the blob region lands in global
key order regardless of thread scheduling, and peak memory is one
batch's output. Side-staged particle-preload meshes drain per batch
into a key-deduped map (first instance wins — per-id extraction output
is deterministic, so instance choice cannot affect bytes) and are
written after all batches, sorted by key, skipping keys already
written.

Program.cs is now a thin arg-parsing shell over the public BakeRunner
so the new dat-gated byte-reproducibility test can drive the REAL
pipeline: tests/AcDream.Bake.Tests (new project, rule 6; registered in
slnx; no Silk.NET in its resolved dependency graph — verified) bakes
the same 9-id mixed fixture twice with DIFFERENT thread counts (8 vs
3 — thread scheduling was the nondeterminism source) and asserts the
two pak files are byte-identical. Ran for real against the dats on
this machine: green.

(10) The isSetup argument for EnvCell extraction now matches at both
call sites (BakeRunner and PakEquivalenceTests both pass false) and is
documented at each: the runtime's own request sites
(WbMeshAdapter.IncrementRefCount / EnsureLoaded) pass isSetup: false
for every MeshRef id including cell-geometry ids. The parameter is
currently dead in MeshExtractor.PrepareMeshData (dispatch is on the
resolved dat type), but two disagreeing call sites were a latent trap.

Header FormatVersion is no longer set by the bake (PakWriter stamps it
per review finding 2, previous commit).

AcDream.slnx

@@ -16,6 +16,7 @@
   </Folder>
   <Folder Name="/tests/">
     <Project Path="tests/AcDream.App.Tests/AcDream.App.Tests.csproj" />
+    <Project Path="tests/AcDream.Bake.Tests/AcDream.Bake.Tests.csproj" />
     <Project Path="tests/AcDream.Content.Tests/AcDream.Content.Tests.csproj" />
     <Project Path="tests/AcDream.Core.Tests.Fixtures.HelloPlugin/AcDream.Core.Tests.Fixtures.HelloPlugin.csproj" />
     <Project Path="tests/AcDream.Core.Tests/AcDream.Core.Tests.csproj" />