CI: Self-host spine count drift guard (582/586 across docs and ROADMAP)

[PurHur]

Problem

M2 spine progress is cited in multiple places with easy drift:

Source	Typical value
test/unit/BootstrapSelfhostBundleTest.php	582 (canonical)
docs/self-host-target.md, README.md, development-status.md	582/586
ROADMAP #78 / #1380 comments	sometimes 578 after hygiene closes
docs/roadmap-wave3.md	582/586 (synced #1815)

Batch hygiene (#1818 ✅) closed stale m2-spine-unit issues but did not add a guard when ROADMAP issue bodies lag BootstrapSelfhostBundleTest.

Goal

1. Add script/check-selfhost-spine-count-sync.php that reads the expected spine count from BootstrapSelfhostBundleTest (or compiler_lib_spine_smoke/main.php require_once count) and fails if key docs diverge:
   • docs/roadmap-wave3.md summary line
   • docs/self-host-target.md M2 table
   • Optional: ROADMAP 🗺️ ROADMAP: Self-host + web-capable PHP subset (living) #78 is manual — document “do not duplicate count in 🗺️ ROADMAP: Self-host + web-capable PHP subset (living) #78; link self-host-target”
2. Gate SELFHOST_SPINE_COUNT_SYNC_GATE in ci-fast.sh (default 0; flip 1 after first sync PR)

Acceptance criteria

php script/check-selfhost-spine-count-sync.php
SELFHOST_SPINE_COUNT_SYNC_GATE=1 ./script/ci-fast.sh
BOOTSTRAP_LIB_SPINE_SMOKE=1 make bootstrap-selfhost-lib-spine-smoke   # still 582 bundle OK

Dependencies

• DevEx: M2 spine open-issue hygiene guard (#1808) #1818 ✅ hygiene script
• Self-host M2: land final 3 ext/standard Vm* units in compiler_lib_spine_smoke (578→586) #1821 — final 3 Vm* units (582→586) will require updating canonical count in one PR + checker

Non-goals

• GitHub Actions
• Replacing php script/bootstrap-inventory.php --check (586 inventory files)

Links

• Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492 · M2 spine growth: batch tracker (compiler_lib_spine_smoke → full inventory) #1419 · ROADMAP 🗺️ ROADMAP: Self-host + web-capable PHP subset (living) #78 Phase 5
• Wave3 doc guard CI: Wave 3 roadmap drift guard (docs/roadmap-wave3.md vs #1380) #1802 ✅ / CI: Default WAVE3_ROADMAP_SYNC_GATE=1 in ci-fast after #1811 doc sync #1814

[PurHur]

claim: implementer-worker-lane-b — starting this run (lane overflow: north-star #32122/#31968 closed; ci-fast red on check-selfhost-spine-count-sync 8042→8043 after #36079 DomCharacterDataJitArgc spine entry)