CI: BOOTSTRAP_PARSER_UNIT_PROBE_GATE opt-in in ci-local after #2409

[PurHur]

Problem

#2409 adds script/bootstrap-selfhost-parser-unit-probe.sh for native M3 link of a CFG/PHPCfg\Parser slice. Without a ci-local gate, the probe only runs when contributors remember it — same gap #2221 / #2361 / #2368 address for Compiler/JIT/VM probes.

Goal

1. BOOTSTRAP_PARSER_UNIT_PROBE_GATE=0 default in script/ci-defaults.env
2. ci_run_bootstrap_parser_unit_probe() in script/ci-common.sh llvm tail (after VM unit probe CI: BOOTSTRAP_VM_UNIT_PROBE_GATE opt-in in ci-local after #2354 #2368)
3. phpc doctor --gates row + docs/local-ci-matrix.md entry
4. Follow-up: default-on flip issue after one release green (mirror CI: Default COMPILER_DRIVER_SMOKE_GATE=1 in ci-local after #2137 opt-in green #2168)

Implementation hints

Piece	Path	Notes
Probe	script/bootstrap-selfhost-parser-unit-probe.sh	From #2409
Env	script/ci-defaults.env	BOOTSTRAP_PARSER_UNIT_PROBE_GATE="${...:-0}"
CI	script/ci-local.sh via ci-common.sh	Skip when LLVM missing
Test	test/unit/CiScriptsTest.php, BootstrapParserUnitProbeTest.php	Assert env documented

BOOTSTRAP_PARSER_UNIT_PROBE_GATE=1 ./script/ci-local.sh

Harness:

make test-harness
./script/docker-exec.sh -- bash -lc 'BOOTSTRAP_PARSER_UNIT_PROBE_GATE=1 ./script/bootstrap-selfhost-parser-unit-probe.sh'

Acceptance criteria

• Gate 0 by default; opt-in 1 does not break default ci-local.sh
• Skip message when LLVM missing
• No .github/workflows/* changes

Dependencies

• Self-host M3: native compile unit probe on lib/Parser.php slice #2409 — probe script + fixture tree (blocker)
• CI: BOOTSTRAP_VM_UNIT_PROBE_GATE opt-in in ci-local after #2354 #2368 — ordering after VM probe in llvm tail
• Self-host M3: compile-driver real lowering symbol batch tracker #1768 — honest lowering (probe should fail if stub-only)

Links

• Epic Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492 · ROADMAP 🗺️ ROADMAP: Self-host + web-capable PHP subset (living) #78 Phase 0 Foundation
• Sibling gates: CI: BOOTSTRAP_COMPILER_UNIT_PROBE_GATE opt-in in ci-local after #2216 #2221, CI: BOOTSTRAP_JIT_UNIT_PROBE_GATE opt-in in ci-local after #2332 #2361, CI: BOOTSTRAP_VM_UNIT_PROBE_GATE opt-in in ci-local after #2354 #2368