perf: Phase 6 — substrate perf-regression alarm (perf-smoke)

Replaces the watchdog-bump approach with an automated check. The next 5× (or
worse) substrate regression will trip the alarm at build time instead of
hiding behind a deadline bump and only being noticed weeks later.

Components:

* lib/perf-smoke.sx — four micro-benchmarks chosen for distinct substrate
  failure modes: function-call dispatch (fib), env construction (let-chain),
  HO-form dispatch + lambda creation (map-sq), TCO + primitive dispatch
  (tail-loop). Warm-up pass populates JIT cache before the timed pass so we
  measure the steady state.

* scripts/perf-smoke.sh — pipes lib/perf-smoke.sx to sx_server.exe, parses
  per-bench wall-time, asserts each is within FACTOR× of the recorded
  reference (default 5×). `--update` rewrites the reference in-place.

* scripts/sx-build-all.sh — perf-smoke wired in as a post-step after JS
  tests. Hard fail if any benchmark regressed beyond budget.

Reference numbers: minimum across 6 back-to-back runs on this dev machine
under typical concurrent-loop contention (load ~9, 2 vCPU, 7.6 GiB RAM,
OCaml 5.2.0, architecture @ 92f6f187). Documented in
plans/jit-perf-regression.md including how to update them.

The 5× factor is chosen so contention noise (~1–2× variance) doesn't trigger
false alarms but a real ≥5× substrate regression — the kind that motivated
this whole investigation — fails the build immediately.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/sx-build-all.sh

@@ -43,4 +43,6 @@ echo "=== JS test build ==="
 python3 hosts/javascript/cli.py --extensions continuations --spec-modules types --output shared/static/scripts/sx-full-test.js || { echo "FAIL: test build"; exit 1; }
 echo "=== JS tests ==="
 node hosts/javascript/run_tests.js --full 2>&1 | tail -3 || { echo "FAIL: JS tests"; exit 1; }
+echo "=== perf-smoke ==="
+bash scripts/perf-smoke.sh || { echo "FAIL: perf-smoke (substrate regressed ≥5×, see scripts/perf-smoke.sh)"; exit 1; }
 echo "=== All OK ==="