feat: trim chunks during generation via step-complete syncTail (CR-6)

The backend now persists chunks at step boundaries during generation
(CR-6). The FE calls syncTail on each step-complete event to fetch
the newly committed chunks. applyHistory clears the provisional array
when new committed chunks arrive mid-generation (they're duplicates
of what was folded from live events). The accumulating chunk (current
in-progress step) is kept.

This means trimTranscript can now drop oldest committed chunks
uniformly during a long turn — no unbounded provisional growth. The
browser never holds more than chatLimit chunks, even mid-generation.

3 new tests: clears provisional on new committed during generation,
keeps provisional when no new chunks, keeps accumulating when clearing.

689 tests green.

0 files changed, 0 insertions, 0 deletions