Fixes:
- message_load() now holds g_message_file_lock for the read, so :last [N]
can no longer observe a half-written line while message_save() is
flushing.
- constant_time_strcmp() accumulates the length difference in size_t.
The old code truncated to unsigned char, which collapsed pairs whose
lengths differed by a multiple of 256 down to 0 and lost the signal.
Refactor:
- buffer_appendf() / buffer_append_bytes() moved to common.c; the two
identical copies in ssh_server.c and tui.c have been removed.
Docs / cleanup:
- README clarifies that exec 'post' uses the SSH login name as the
author and that anonymous mode performs no identity check.
- Removed TODO.md (both items completed) and docs/README.old.
- Trimmed the auto-generated 2025 entry block from docs/CHANGELOG.md
and added a 2026-05-16 entry summarising this change.
