Address findings from the four-agent review of PR #298:
- file_create: zero the new File.matched_bytes. It was uninitialized
malloc memory, so the receiver could sum a garbage value into
STATUS_STATS "Matched data" (nondeterministic --stats divergence and
an uninitialized-heap disclosure on the wire).
- send_append: load the source into memory before hashing/copying the
prefix and tail. Files >64 MiB without compression (and --sendfile
runs) are streamed without loading, so --append/--append-verify
dereferenced a NULL data->data and crashed.
- filter_file_append: clamp the rollback to the surviving rule count.
A "clear" rule in a merge file frees every rule including the
caller's; the old rollback rewound count to rules_before and
resurrected freed pointers for a double free / UAF. Also roll back
when set_rule_owner fails instead of leaving owner-less rules.
- filter_rule_parse: reject the xattr-name filter modifier (x), which
was parsed and silently reinterpreted as a filename rule (affecting
what --delete protects). The p modifier stays accepted (existing
grammar test).
- Remove two dead functions: compression_default_algo and
change_render_itemize_code.
- tests: free ctx->would_delete in the two test_multiprocessing manual
teardowns (ASan leak, 1648 bytes/run).
- docs: correct the RSYNC_COMPAT/HANDOFF tally (156 rows: 106/27/23),
downgrade --info/--debug to caveat with their silent categories, add
%C-vs-xxh64 and --delete-delay count caveats, refresh stale xattr
mode comments, and document -p special-bit (setuid/setgid/sticky)
parity plus its mitigations.