After a poll prunes runs, a branch quiet for 30+ days shows an older run's Success (or a stuck Pending) instead of its newest run's Failure #418

What's wrong

Build.PruneRuns (BuildMonitor/Build.cs:91-125, added in #398 and called from BuildSync.UpdateBuildAsync after every build poll) applies each keep-rule to runs one at a time, not per branch:

• "newest run on a branch" applies only to branches whose newest run is within BranchRetention (30 days);
• "ongoing" and "the estimator's newest successful runs" keep runs regardless of branch.

So for a branch outside the 30-day window, its newest run can be removed while an older run on the same branch survives through one of the other rules. RenderBuildTable (BuildMonitor.cs:620-625) groups the remaining runs by branch and shows the newest one left. That is now the older run. The status filter (ShouldShowBuildBranch), History and Errors read the same wrong run. ListByWorkflow only returns the newest runs, so the dropped run never comes back, and the result is persisted to the app data file.

The PruneRuns remarks and CLAUDE.md say "A branch none of whose runs is kept disappears from the table". Here the branch stays, showing a run that isn't its latest.

Repro (MSTest, in the style of RunRetentionTests, Now = 2026-06-01)

• release/1.0: rel-older at Now−40d (Success), rel-newest at Now−35d (Failure)
• 60 Failure runs on branches pr-0…pr-59, one hour apart, ending at Now
• build.PruneRuns(Now)

Why it matters

A failed release branch turns green in the monitor, with no new run, just because other branches were busy. A failure is hidden, which is the one thing a build monitor must not do. In the Pending case, the row shows a progress ring for a build that finished long ago.

Suggested fix

Decide per branch. After building kept from the existing rules, add each kept run's branch-newest run, so a branch either shows its true latest run or disappears:

HashSet<string> keptBranches = [.. runs.Where(r => kept.Contains(r.Id)).Select(r => r.Branch)];
kept.UnionWith(runs.Where(r => keptBranches.Contains(r.Branch))
    .GroupBy(r => r.Branch)
    .Select(g => g.MaxBy(r => r.Started)!.Id));

Acceptance criteria

• With the repro above, release/1.0's newest remaining run is still rel-newest (Failure), in both the Success and the Pending variant.
• A regression test is added to RunRetentionTests, and the existing retention tests still pass.