dita_v2: VenueTelemetrySnapshot + Zinc venue plane (4th shm partition) + local Rust target dir

Upstream-canonical commit of the ~366 lines that sat uncommitted in the working
tree (surveyed in DITAV2_SOA_SURVEY_20260702). Venue telemetry published at every
venue boundary (submit/cancel/reconcile) over the zinc venue_region; SOA per
UV DITAv2 adjudication 2026-07-03.

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

prod/clean_arch/dita_v2/rust_backend.py

@@ -107,6 +107,9 @@ def _crate_dir() -> Path:
     return Path(__file__).resolve().with_name("_rust_kernel")
 
 
+_LOCAL_TARGET_DIR = Path("/root/.cargo/dita_v2_target")
+
+
 def _library_path() -> Path:
     if sys.platform == "darwin":
         name = "libdita_v2_kernel.dylib"
@@ -114,6 +117,9 @@ def _library_path() -> Path:
         name = "dita_v2_kernel.dll"
     else:
         name = "libdita_v2_kernel.so"
+    local = _LOCAL_TARGET_DIR / "release" / name
+    if local.exists():
+        return local
     return _crate_dir() / "target" / "release" / name
 
 
@@ -121,10 +127,13 @@ def _build_library() -> None:
     crate_dir = _crate_dir()
     if not crate_dir.exists():
         raise FileNotFoundError(f"Missing Rust kernel crate: {crate_dir}")
+    _LOCAL_TARGET_DIR.mkdir(parents=True, exist_ok=True)
+    env = {**os.environ, "CARGO_TARGET_DIR": str(_LOCAL_TARGET_DIR)}
     subprocess.run(
         ["cargo", "build", "--release", "--manifest-path", str(crate_dir / "Cargo.toml")],
         cwd=_repo_root(),
         check=True,
+        env=env,
     )