CI: check-capabilities-throws-sync.php drift guard (007 / #195 rows)

[PurHur]

Problem

#2103 asks for curated throw / try / catch rows in docs/capabilities-syntax.md, but there is no drift guard like #2068 (check-development-status-sync.php) or #2019 (006 multipart rows). Doc-only PRs can claim VM/JIT/AOT yes while ROADMAP #78 still lists #195 open.

Goal

1. Add script/check-capabilities-throws-sync.php that asserts:
   • docs/capabilities-syntax.md contains a try / catch / throw row with expected issue links (Language: try/catch/finally lowering #57, Language: throw expressions and Exception object propagation #195, Testing: try/catch compliance PHPT pack (phase 1 of #57) #2084)
   • Optional curated 007-ThrowsWeb web footnote block (mirror sessions/file-upload sections in script/capability-syntax-lib.php)
2. Wire opt-in gate CAPABILITIES_THROWS_SYNC_GATE=0 in script/ci-defaults.env
3. Hook in script/ci-common.sh ci_run_inventory_checks() (same band as WAVE3_ROADMAP_SYNC_GATE)
4. Document in docs/local-ci-matrix.md
5. Follow-up issue to default-on in ci-fast after Docs: capabilities-matrix throw / try / catch rows (post-#195/#57) #2103 lands

Implementation hints

File	Notes
script/check-capabilities-throws-sync.php	Regex/line checks; exit 1 + hint to run php script/capability-syntax.php
test/unit/CapabilitySyntaxTest.php	Assert gate env + script exists
script/capability-syntax-lib.php	Source of truth for curated web row text

Acceptance criteria

php script/check-capabilities-throws-sync.php
CAPABILITIES_THROWS_SYNC_GATE=1 ./script/ci-fast.sh --filter CapabilitySyntax

Dependencies

• Docs: capabilities-matrix throw / try / catch rows (post-#195/#57) #2103 — matrix content landed first (or same PR)
• Docs: regenerate capabilities-syntax.md (ci-fast CAPABILITY_SYNTAX_CHECK stale on master) #2129 — regen capabilities-syntax.md on master

Verification

./script/docker-runforge.sh -- bash -lc 'php script/check-capabilities-throws-sync.php'

No GitHub Actions (#394 ✅).

Links

• Docs: capabilities-matrix throw / try / catch rows (post-#195/#57) #2103 · Docs: capabilities-matrix multipart/ + 006-FileUploadWeb AOT status rows #2019 pattern · ROADMAP 🗺️ ROADMAP: Self-host + web-capable PHP subset (living) #78

[PurHur]

Implemented in #2148. Local Docker verification: check-capabilities-throws-sync.php, capability-syntax.php --check, and targeted PHPUnit green. Gate stays opt-in (CAPABILITIES_THROWS_SYNC_GATE=0) until default-on follow-up after #2103.