DYADT verifier: reject always-matching `contains:` patterns + minor spec/coverage gaps

[hyperpolymath]

Child of #460. A 3-lens adversarial review of the DYADT reference verifier (scripts/verify-claims.sh) confirmed 16 holes; 15 were fixed in the Wave-4 hardening (#458). This tracks the residual + minor items.

Residual (real, must fix)

Always-matching contains: regex vacuously confirms. The hardening rejects an empty contains: pattern, but not a trivially-always-matching one. expect="contains:.*" (also ^, $, ., .+) passes grep -Eq against any non-empty file → confirmed for content the agent never added.

• Fix: require a contains: pattern to contain at least one literal (non-metacharacter) character — reject if, after stripping regex metachars/quantifiers, nothing remains. Add a conformance vector (always-match.a2ml → unverifiable).
• Note: stdout-contains: uses fixed-string grep -F, so .* there is a literal search and is NOT vacuous; only the regex contains: is affected.

Minor (spec clarity / coverage)

• Regex dialect unspecified (contains:<regex>): the reference uses POSIX ERE (grep -E); two conforming verifiers could disagree. Pin the dialect in CLAIM-FORMAT.adoc.
• Over-broad licence guard (LOW, accepted): any claim whose fields contain licence/license/SPDX → unverifiable manual-only. This is fail-safe (a benign claim flagged manual-only is harmless per the Manual-Only policy), but could be narrowed to claim_class/an explicit flag later.
• Conformance coverage: several normative MUSTs still lack a vector. Add vectors so the production verifier is fully constrained.

Acceptance: the always-match hole is closed with a regression assertion + conformance vector; the spec pins the regex dialect.