feat(stovepipe): report last-green freshness - #572

## 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 uber#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 uber#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. uber#572
1. @ uber#573