CI: enforce the stability policy's Rust-API row with cargo-semver-checks

docs/stability.md promises the Rust API surface is "Semver-checked in CI (planned: cargo-semver-checks)" — nothing tracks that plan. Add a CI job running cargo-semver-checks against the latest published release for the published crates (awa, awa-model, awa-macros, awa-metrics, awa-worker, awa-testing, awa-seaorm), advisory on PRs and blocking on release branches. Makes the #369 policy enforceable rather than aspirational; pairs with the release checklist in the #383 tracker.

Read docs/stability.md and the release checklist referenced by issue #383, then inspect the repository's existing CI configuration. Determine how cargo-semver-checks should compare each listed published crate with the latest release, and how CI distinguishes pull requests from release branches. Done means all seven crates are checked, PR checks are advisory, and release-branch checks block failures.

Written by the indexing model from the issue text.