feat(stovepipe): report build failure detection latency

[mnoah1]

Summary

Emit how long a break went undetected: when a validation build fails, record the elapsed time from the commit timestamp of the base it validated against, as a histogram tagged with the queue and the build strategy. A histogram rather than a gauge because the distribution over failures is the point — what an operator wants is how long a break typically survives, not how long the last one did. The buckets (ChangeAgeBuckets) span minutes to a month, since a break caught in minutes and one that survived a fortnight are both ordinary observations.

The observation lives in buildsignal, the stage that records the failure. A failure is the moment a break becomes known, and an elapsed time is only meaningful against it, so unlike the last-known-green age in #572 there is no later moment to sample it from — it cannot be moved off the delivery path onto the periodic schedule. The cost is a source-control call on that path, so it is confined to failures, made after the outcome is durable, and swallows every fault: a failed observation is counted with the step that failed and never disturbs the outcome already written.

A full build pins no base commit, so its failures have nothing to measure from. That is the ordinary case for the strategy rather than a fault, so those failures are counted as unmeasurable (detection_missing) instead of landing in the error series.

Rebased onto the restructured #572, so the emits follow the same conventions as the last-green observation there: an operation name for what is measured rather than for the stage, detection_errors tagged with the step that failed, a separate counter for "nothing to measure", and failures logged. The source-control factory is a required constructor dependency and wired in service/stovepipe/server/main.go.

Test Plan

• make test — table-driven unit tests cover the measured path, the no-baseline (full build) case, and every step that can fail to observe: source control not resolving, ChangeInfo failing, an undated change, and a change dated in the future.
• make lint, make check-gazelle, make check-tidy, make build.
• Deploy and add a query for build_failure.time_to_detection, confirming detection_errors stays flat and detection_missing tracks only full-build failures.

Issues

Stack

1. feat(stovepipe): report last-green freshness #572
2. @ feat(stovepipe): report build failure detection latency #573