fix(policy): fail closed when set_check extraction breaks

[brenpike]

Summary

tools/policy_check.sh test_set_check could not tell a broken regex extraction apart from a genuine zero-match result: both collapsed to an empty capture {}. A zero-count subset fixture (notably tests/policy/safety-destructive-fix-gate-no-second-copy.json) could therefore pass vacuously if its extractor ever broke. This PR makes the check fail closed.

Closes #373.

What changed

• Regex passed as data. The fixture regex now reaches perl through the RE environment variable and is compiled with qr//. It is never spliced into program source, so an unescaped / can no longer break the program.
• Same engine validates and executes. The compile check now uses perl qr// instead of grep -P, a different engine.
• Exit status checked separately. The extractor's exit status and the jq aggregation's exit status are each checked, and each has its own finding. A match with no group-1 capture now fails. {} can only come from a successful extraction that found nothing.
• perl is a hard dependency. The grep -oP / sed -E fallback is deleted. It could not parse the PCRE (?:...) syntax the fixtures use, and it returned zero captures silently. If perl is missing, the check now fails loudly and names perl. perl is an essential package on the ubuntu-latest CI runner, and the script already depends on grep -P elsewhere with no fallback.
• Canary coverage. The in-script set_check SAFETY-CANARY adds three assertions: a non-compiling regex fails, a regex that matches but has no capture group fails, and a regex containing an unescaped / passes and captures a/b.
• Stale citation replaced. tests/policy/README.md line 10 cited tools/policy_check.sh:1473, which was already stale. It now points at the SAFETY_FIXTURES discovery block by name. This is backlog item 6 of the prompt-audit follow-ups, folded in because this PR already edits the README.

Files by plan step

• STEP-001: tests/policy/fixtures/set-check-zero-match-canary.md (slash marker line, prose describing the five canary branches)
• STEP-002: tools/policy_check.sh (test_set_check rewrite, canary assertions 3-5)
• STEP-003: tests/policy/README.md (new section 8 "Runtime dependencies", line 10 pointer)

Validation

• bash tools/validate.sh --changed escalated to the full suite plus --self-test because tools/** changed. Exit 0, all suites pass.
• policy_check.sh --strict: Checks passed 72/73, 0 new findings (unchanged baseline).
• An old-vs-new harness confirmed that the no-capture and non-compiling cases previously passed vacuously and now fail. The legitimate zero-match case passes in both.
• A mutation test (reverting to || extract_out="") makes the canary fail, so the canary catches the regression it guards.
• All 9 existing set_check fixtures keep identical verdicts.
• Local Codex adversarial review: clean on the first iteration, zero findings.

Versioning

No bump. Only tools/ and tests/ changed; no plugin/ file is touched.

Unresolved

None. The script-wide discovery gaps in other find sites remain tracked in #377.

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