Replace per-contact directory structure with a single flat file per contact.
Each file uses pipe-delimited records with a sparse byte-offset index
that is rebuilt on demand and cached in memory.
- Rewrite _ff_write_record / _ff_read_records for single-file format
- Add sparse index (every Nth record) for O(log N) seek on history read
- Cursor-based pagination: _ff_get_previous_chat uses saved byte offset
- LMC corrections applied in-place during read (_ff_apply_lmc)
- Newline escaping (\n) in message bodies for line-based storage
- Simplify verify: parse + timestamp order + LMC reference check
- Update parser helpers for new field layout
