TBv3: repository summary shows the previous version's numbers after switching versions

[paynejd]

Summary

After you switch to another version of the same repository, the summary panel shows the previous version's numbers under the new version's heading until the new summary loads. The summary request takes about 8 to 11 seconds. If the previous version's request finishes last, its numbers stay until a reload.

Steps to reproduce

1. Open CIEL HEAD and wait for the summary panel to fill in.
2. In the same tab, switch to v2026-09-24.
3. For several seconds the panel reads "CIEL:v2026-09-24 Summary: 55,583 active concepts (3,290 retired), 333,277 active mappings". Those are HEAD's numbers; the release has 3,287 retired concepts and 333,274 active mappings.

If you switch before HEAD's summary has loaded and HEAD's request finishes last, the HEAD numbers stay.

Acceptance criteria

• While a version's summary loads, the panel shows its loading state, not another version's numbers.
• A late response for a version you've left never replaces the current version's summary.

Notes

fetchRepoSummary in src/components/repos/RepoHome.jsx doesn't clear repoSummary before fetching, and doesn't check that the response is for the version still on screen. Moving to another repository is covered by #2819, where RepoHome now starts fresh per repository. This is the same-repository case. It's a different problem from #1805, which is about the API computing wrong counts after imports.

Related: #2819.