Add len(b.data) > 0 guard to prevent infinite loop when an empty linked
buffer is reachable. With the guard false, the normal path handles empty
buffers safely via the existing t < b.start || idx >= len(b.data) check.
Add regression test TestStatsFastPathThenPartialTail to pin the boundary:
three-buffer chain where earlier buffers are fully covered (fast path) and
the last buffer is partially covered (normal path).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Aditya Ujeniya