fix(metrics): stepId-based segment matching — robust against chat-limit trimming

When the chat limit trims old chunks, head-aligning turn metrics to
segments by position breaks (a trimmed user message removes a segment
boundary, shifting all subsequent alignments by one). Fix: match segments
to metrics entries by stepId overlap (pass 1), falling back to sequential
matching for text-only segments with no stepId-bearing groups (pass 2).

This prevents step/turn metrics from being placed on the wrong segment
after trimming, while preserving the original behavior for text-only turns.

686 tests green.

0 files changed, 0 insertions, 0 deletions