Give the Rust CultMesh runtime a home

The only Rust CultMesh implementation in existence was 1,363 lines inside
Ghostlight's vendor directory. It was in no library repo and no standalone
repo, so it had never been checked against the browser, Kotlin, and
TypeScript CultMesh bodies it must interoperate with.

It is not a duplicate of the facade in cultnet-rs. That one is the small
surface the parity table claims for Rust: peer cards, authority leases,
endpoint parsing. This is the stream and session layer -- node, document
sets, RUDP document publication, stream descriptors, negotiation, stream
catalog, and shared-memory frame rings -- which the same table lists under
Rust's "Not claimed". A consumer grew the unclaimed surface because there was
nowhere else to put it.

Its path dependencies already matched this layout and it builds against
CultLib's cultnet-rs unchanged, so it does not depend on the forked RUDP
extras. Seven runtime-local tests pass, and CI now runs them.

The parity table gains no claim from this. The Rust row says plainly that no
interop lane exercises the stream surface, and expansion work records that
runtime-local tests are not a parity claim. Landing code is not evidence.

packages/cultmesh-rs needed an explicit .gitignore re-include; the NuGet
packages rule hides every language package until one is added by name.

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

.github/workflows/cultnet-interop.yml

@@ -92,6 +92,9 @@ jobs:
       - name: Verify Rust RUDP reliability
         run: cargo test --manifest-path packages/cultnet-rs/Cargo.toml rudp
 
+      - name: Verify Rust CultMesh runtime-local surface
+        run: cargo test --manifest-path packages/cultmesh-rs/Cargo.toml
+
       - name: Verify C# RUDP reliability
         run: dotnet test tests/GameCult.Networking.Tests/GameCult.Networking.Tests.csproj --filter "FullyQualifiedName~Rudp"