A fork that is ahead of its parent shows "Offer N commits upstream" on its home, opening the cross-repository pull form already filled in #5964

Good changes bubble up as offers. A fork should not have to find the compare view to offer its work.

Done when:

• [ ] On a fork's home (src/web/pages/repo.ts), when forkDrift says it is ahead of its parent, a button under the head reads Offer 3 commits upstream. It opens the existing new-pull form with the fork's default branch as head and the parent's default branch as base, both filled in.
• [ ] It is shown only to viewers who may open a pull on the parent, and only when no open pull already offers that branch. When one does, the line links to it instead: Offered in #N.
• [ ] The drift read uses the stored figures, never git on render, and is a named Server-Timing phase.
• [ ] Tests: ahead with no pull, ahead with an open pull, level, and a viewer who may not open a pull. Do not touch src/http/mcp.ts, src/contract/openapi.ts or docs/DESIGN.md.