Harden the gate against corpus-set drift: vacated-id resurrection and re-added files pass green

[systemfsoftware-maker]

Problem

Two accepted-by-design narrowings leave the one-file gate green through states the corpus law forbids:

1. Resurrecting a vacated id passes. Adding a rule with id CONST-T1 (or T2/T5, G1/G2, E1-E4) today yields valid: 41 rules and exit 0. AGENTS.md forbids resurrecting vacated ids forever; the gate's docstring states "Vacancy is named, not enforced" — the enforcement surface is review only.
2. A re-added corpus file is invisible. PATHS = ["CONSTITUTION.md"] is hardcoded; re-adding a CONSTITUTION-ARTICLES.md (or any second law file) is not a defect to the gate — the vacuous-pass doctrine was scoped to the declared input set, and the set itself can drift silently.

Why filed, not fixed

Both were settled design decisions for this branch (KTD6: the narrowing is the contract; vacancy naming is the check). Hardening is real governance design — a hardcoded VACATED_IDS set goes stale the next time a rule is vacated; wiring --against HEAD~1 into every pre-commit changes the repo's verification law beyond this change's scope.

Suggested fix

Either (a) a VACATED_IDS set in scripts/validate-constitution.ts that fails on mint-collision with historical ids (self-maintaining: add ids as they are vacated), or (b) pre-commit runs deno task test --against HEAD~1 and treats reassigned id: lines as failures, plus a glob step failing on any second CONST*.md corpus file at repo root.

Severity: P2 (advisory)
Confidence: 100 (scenario constructible), 75 (fix shape)
Reviewer(s): adversarial (corroborated by testing on the fixture-persistence half)
Source: branch restore-single-document, commit 0e853af; ce-code-review run 20260901-050207-ecc9e7b4

Source: PR #21 (#21)