CI: BOOTSTRAP_M4_LOOP_PROBE opt-in in ci-local after #1498 probe green

[PurHur]

Problem

#1498 defines M4 — native binary from revision N compiles sources and produces a working N+1 binary without Zend php bin/compile.php for that step. script/bootstrap-loop-probe.sh exists (dry-run opt-in) but there is no ci-local hook — regressions in the bootstrap loop are invisible until manual BOOTSTRAP_LOOP_PROBE=1 runs.

M3 strict emit (#1937) and M2 spine (#2050) must stay green before M4 promotion.

Goal

When script/bootstrap-loop-probe.sh passes on php-compiler:22.04-dev:

1. Add BOOTSTRAP_M4_LOOP_PROBE="${BOOTSTRAP_M4_LOOP_PROBE:-0}" to script/ci-defaults.env
2. ci_run_bootstrap_m4_loop_probe() in script/ci-common.sh — llvm tail only; skip when LLVM missing (exit 0)
3. Wire into script/ci-local.sh after M3 HelloWorld strict probes
4. Follow-up issue: default-on flip (mirror CI: Default BOOTSTRAP_M3_HELLOWORLD_STRICT_GATE=1 in ci-local after #1493 #1866 pattern)

Implementation hints

Piece	Path	Notes
Probe	script/bootstrap-loop-probe.sh	--dry-run today; strict mode when #1937 lands
Fixture	test/selfhost/bootstrap_loop_smoke/	From #1498
Env	BOOTSTRAP_LOOP_PROBE_GATE vs BOOTSTRAP_M4_LOOP_PROBE	Align naming with docs/bootstrap-m5-fast-path.md
Doctor	phpc doctor --selfhost (#2053)	List M4 probe env

Acceptance criteria

BOOTSTRAP_M4_LOOP_PROBE=1 ./script/ci-local.sh --filter BootstrapLoop
docker run --rm -v "$(pwd):/compiler" -w /compiler php-compiler:22.04-dev \
  BOOTSTRAP_M4_LOOP_PROBE=1 ./script/ci-local.sh --filter BootstrapLoop

No GitHub Actions required.

Dependencies

• Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492 epic · M4: Bootstrap loop — native binary rebuilds next compiler revision #1498 M4 definition · Self-host M3: Native emit for compiler compile-smoke fixture (post-#1493) #1937 strict emit (recommended before default-on)
• M5: Prelink vendor (php-cfg, php-types, php-llvm) — no composer at cold boot #1416 vendor prelink (M5 — orthogonal to opt-in probe)

Links

• docs/bootstrap-m5-fast-path.md · ROADMAP 🗺️ ROADMAP: Self-host + web-capable PHP subset (living) #78 Phase 5 Self-host