feat(correlation): unify Railway correlation seeding with shared clustering

## Parent

#5981

## What to build

Make Railway-generated correlation cards use the same exact entity clustering, display, identity, and runtime-mode decisions as the browser.

## Acceptance criteria

- [ ] Replace the seeder's hand-copied clustering behavior with the shared clustering contract.
- [ ] Mirror every transitive shared dependency required by the Railway scripts-only bundle and enforce byte identity.
- [ ] Prevent repository-root shared imports from escaping into the Railway container.
- [ ] Prove browser/seeder parity for clustering, display suppression, runtime modes, and failure fallback.
- [ ] Keep `legacy` mode output unchanged and preserve existing seeder trend-state behavior.

## Blocked by

- #5983
- #5984

Start by tracing the Railway correlation seeder and the browser's shared clustering contract, after #5983 and #5984 are resolved. Compare clustering, display suppression, identity, runtime modes, failure fallback, and legacy output, then verify that the scripts-only bundle has byte-identical dependencies without repository-root imports escaping the container.

Written by the indexing model from the issue text.