Adds opt-in disk persistence to SpoolStore. `enable_persistence(dir)`
turns every enqueue into a single `<seq>.spool` file alongside the
in-memory queue; drain and clear delete the matching files; restart
replays the directory sorted by seq.
Writes are atomic: serialize the message via the SECS-II codec, write
to `.tmp`, then `std::filesystem::rename` to the final name. Malformed
records are dropped silently so a single bad file can't poison the
whole spool.
`secs_server --spool-dir <path>` enables persistence at startup.
Without the flag the behaviour is identical to before (in-memory only).
Two new tests: enqueue → restart → replay → drain restores the wire
order, and clear deletes the journal files.
Test suite: 291 cases / 1515 assertions.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
raphael
d69f26b415