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fix(bb_ring): clamp bytes_used against u32 underflow on empty/drifted ring (B1-473) (#661)

- bytes_used was decremented unconditionally on pop/evict with no floor;
  accounting is symmetric by construction today, but any future drift (or
  a malformed entry) could wrap size_t to ~4.29e9 once narrowed to uint32_t
  by bb_ring_diag, surfacing as garbage on /api/diag/rings.
- add bb_ring_bytes_used_sub() helper used at both decrement sites
  (push-evict, pop_oldest); clamps to 0 instead of wrapping.
- add BB_RING_TESTING hook (bb_ring_test_force_bytes_used) to inject
  drift and exercise the clamp branch in host tests.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

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