fix(metrics): tail-align when stepId matching fails — prevent misaligned turns

When stepIds are absent on persisted chunks (or don't match), the
sequential fallback was HEAD-aligning — matching the OLDEST entries
(trimmed turns) to the NEWEST segments. This showed 'turn 1' on turn 20's
content and placed the wrong metrics on the wrong segments.

Fix: when stepId matching produces ZERO matches, use TAIL-ALIGNMENT
instead — match the LAST T entries to the T segments (the loaded chunks
are always the newest). The oldest entries (trimmed turns) are unmatched
and emit standalone turn-metrics rows at the top.

686 tests green.

0 files changed, 0 insertions, 0 deletions