M3: Quality substrate

Drafted with AI assistance; reviewed by @andunn.

Tracks milestone M3 of the plan epic. A merged MR advances a checkbox; this tracker closes only on maintainer review of the whole milestone.

Verdict quality becomes measurable and drift becomes visible.

• A rule-family-stratified eval corpus (a deliberate subset of the full control set, growing as families are exercised) is built, published, and baselined
• A scheduled heartbeat pipeline runs: parser canary at the pin, single-rule smoke against a pinned fixture, rule-pack provenance verification, and an informational pin-versus-latest report, with failures auto-filing issues
• An input-contract stability ADR is adopted: additive-only inputs within a major version, a deprecation window for removals

Part of &1