fix(metrics): preserve turn-metrics for trimmed turns

When the chat limit unloads a turn's content (user message + chunks),
the segment disappears and the turn-metrics row was lost. Now unmatched
entries (fully trimmed turns) emit a standalone turn-metrics row at the
top of the transcript, so the user still sees 'turn N · X tok' for
unloaded turns.

Note: trimming during generation only affects COMMITTED chunks (old
turns). Provisional chunks (the in-flight turn) are never trimmed — the
big trim happens at seal when provisional → committed. This is by design.

0 files changed, 0 insertions, 0 deletions