Scaffolding for the canary-model rework (host-thread-per-guest-thread,
opt-in, non-deterministic). No behavior change with the flag off.
- XENIA_NATIVE_THREADS=1 selects the new executor; folded into the
parallel spawn gate (reuses Arc<Mutex<KernelState>> + worker dispatch).
- native_threads_enabled() = single source of truth for guarded branches.
- run_execution_native(): Stage-0 body delegates to the iterate-4D
free-run executor verbatim (native == freerun for now); Stage 1' will
replace the worker set with one host thread per guest thread.
- native-gate.sh: functional oracle replacing byte-goldens for the MT
path — (1) lockstep golden byte-identity (flag-off safety net),
(2) native render milestone (draws>0 && swaps>0), (3) native deadlock
stress. GREEN 3/3 on current code.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
