bitbucket: Implement ChecksByChange with per-status build detail

[ed-irl]

Implements the Bitbucket side of the per-change checks API: rollup
state from the PR head commit's build statuses, plus per-status
name/state/url forwarded as runs[].

• gateway: CommitStatus grows Key, Name, and URL fields. Existing
  callers and fixtures keep working because the new fields are
  omitempty.
• rollupFromCommitStatuses collapses Bitbucket build-status
  states to the upstream rollup taxonomy. Failure wins over
  pending wins over passed. Empty list means no CI configured
  -> ChecksNone. Stopped statuses are operator-cancelled
  non-failures and roll up as passed (changed from the merge
  handler's aggregateStatuses, which treated Stopped as Failed;
  that path is preserved for backwards compatibility but no
  longer informs the richer API).
• ChecksByChange issues getPullRequest + CommitStatusList per
  PR. State is lowercased to match the spec's forge-native
  convention; Name falls back to Key when the build-status
  provider didn't supply a human name.
• aggregateStatuses (the merge handler's path) keeps its
  pre-existing semantics and is now scoped narrowly so the
  refactor doesn't change merge behavior.

Integration fixtures regenerate on the next forgetest -update
pass (real Bitbucket credentials).

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • forge: Add ChecksByChange and ChecksNone for per-change checks output #1263
    • shamhub: Implement ChecksByChange with full rollup + runs + URL seeding #1264
      • github: Implement ChecksByChange with per-run detail and checks URL #1265
        • gitlab: Implement ChecksByChange with per-job pipeline detail #1266
          • bitbucket: Implement ChecksByChange with per-status build detail #1267 ◀
            • list: Emit per-change forge checks via --cr-checks JSON flag #1268
              • docs: Document forge checks JSON output and add changelog entry #1269

_{Change managed by git-spice.}