Module write_path 

Append-only write logic for the RVF runtime.

All mutations append new segments to the file. The write path:

1. Allocate segment_id (monotonic counter)
2. Build payload (VEC_SEG, META_SEG, JOURNAL_SEG, etc.)
3. Write segment header + payload, fsync
4. Build new MANIFEST_SEG, fsync (two-fsync protocol)