feat(keel-repin): repin shared kit and engine layers

[sylphx-desk-studio]

What changed

Extend the existing keel-repin action with layer: kit and layer: engine, leaving the default verified Keel tag flow unchanged.

• Poll the selected source's main commit and open one deterministic PR per target, without downgrading consumer pins or rebuilding an existing branch.
• Move cubeage-kit and the Keel revision it pins as one tuple, using the existing two-argument repin hook or the default Cargo/revision-file rewrite and lock refresh. A kit lock must contain one Keel source at the target revision.
• Move tycoon-engine Cargo pins and ENGINE_REV before invoking the title's vendor hook, with a fetched ENGINE_REPO checkout available to local-export hooks.
• Keep the same consumer-CI, exact-head settle gate. Failed builds open owned drafts. New layer PRs supersede ancestor PRs of the same layer only.
• Document the inputs and add offline git/Cargo/title-hook fixtures, including duplicate suppression, supersession, no downgrade and repeated red settle conditions.

Why

Shared kit and engine updates currently need hand-built consumer branches. Keeping them inside the existing repin action preserves the consumer's build and CI gates, while moving the kit's Keel pin together avoids a split Keel dependency graph.

Testing

• A remote Build run of the initial shared-layer fixtures passed: 2 tests in 4.500 seconds. It exercised real local git main commits, a real Cargo kit/Keel lock resolution with one Keel revision, both engine vendor-hook contracts, one PR on repeated polls, and failed-build drafts mentioning their owner.
• git diff --check origin/main HEAD passed on the final head.
• Final repin regression coverage and full python -m unittest discover -s tests remain for CI. The remote full-suite attempt could not acquire a machine within 120 seconds; the final repin-only attempt could not start because every workspace was in use. Neither attempt ran tests or returned a test failure.
• No Rust, migration, or generator paths changed.

[sylphx-desk-services]

Reviewed the kit/Keel tuple rewrite, engine hook flow, duplicate suppression and exact-head settlement. Shared-layer rewrites now preserve unrelated Keel release tags and comments; the added verified/weekly fixtures cover both engine hooks, lock and pin preservation, and owned failed-build drafts. Tests and ci-ok succeeded on this head. No blocking findings. The merge writer arms this same head through the normal merge path.