Self-host M3: check-m3-allowlist-snapshot.php drift guard (#1768 SSOT)

[PurHur]

Problem

#1768 tracks M3 compile-driver symbol batches in lib/JIT.php (isM3CompileDriverRealLoweringName(), isM3CompileDriverSpineDenyName()), but the allowlist is hand-edited with no CI drift guard.

#1872 ✅ solved the same problem for M2 spine 589/593 counts via script/bootstrap-spine-count.php + check-selfhost-spine-count-sync.php. M3 batches need an analogous snapshot so PRs don’t silently shrink or duplicate allowlist entries.

Goal

1. script/check-m3-allowlist-snapshot.php — parse lib/JIT.php allowlist/denylist names, compare to committed script/m3-allowlist-snapshot.txt (or JSON)
2. M3_ALLOWLIST_SYNC_GATE=1 in ci-fast inventory checks (default on; opt-out =0 for bulk symbol PRs)
3. Self-host M3: compile-driver real lowering symbol batch tracker #1768 issue body table updated by regenerating snapshot in each batch PR

Implementation hints

Piece	Path	Notes
Generator	script/bootstrap-m3-allowlist-snapshot.php	rg or PHP parse of isM3CompileDriverRealLoweringName array bodies
Checker	script/check-m3-allowlist-snapshot.php	Exit 1 with diff hint: run generator
Snapshot	script/m3-allowlist-snapshot.txt	One name per line, sorted
ci-common	script/ci-common.sh	Wire next to check-selfhost-spine-count-sync.php
Test	test/unit/M3AllowlistSnapshotTest.php	Assert checker script exists

php script/bootstrap-m3-allowlist-snapshot.php --write
./script/check-m3-allowlist-snapshot.php
M3_ALLOWLIST_SYNC_GATE=0 ./script/ci-fast.sh   # bulk PR escape hatch

Acceptance criteria

• Drift between lib/JIT.php and snapshot fails ci-fast with regenerate instructions
• Self-host M3: compile-driver real lowering symbol batch tracker #1768 batch PRs include snapshot update in the same commit
• Documented in docs/bootstrap-m5-fast-path.md M3 section

Verification (local / Docker only)

./script/ci-fast.sh --filter M3Allowlist
docker run --rm -v "$(pwd):/compiler" -w /compiler php-compiler:22.04-dev \
  ./script/ci-fast.sh --filter M3Allowlist

No GitHub Actions.

Dependencies

• Self-host M3: compile-driver real lowering symbol batch tracker #1768 — umbrella batch tracker
• M3: BOOTSTRAP_M3_HELLOWORLD_STRICT — native HelloWorld emit (no Zend fallback) #1493 — strict HelloWorld emit (consumer of allowlist growth)
• Self-host: Single source of truth for M2 spine unit counts (584/585/586/588 drift) #1872 — precedent SSOT pattern

Links

• Epic Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492
• ROADMAP 🗺️ ROADMAP: Self-host + web-capable PHP subset (living) #78 Phase 5 Self-host