fix: review pass — uninit stats, append crash, filter rollback, ASan leak

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.

src/shared/compression.h

@@ -22,8 +22,6 @@ typedef enum {
   COMPRESSION_ALGO_ZLIBX = 4
 } CompressionAlgo;
 
-CompressionAlgo compression_default_algo(void);
-
 /* Resolve a --compress-choice string (case-insensitive) to an algorithm id.
  * Accepts "zstd", "lz4", "zlib", "zlibx", "none".  "auto" is not an algorithm
  * here; the caller resolves it to the negotiated default.  Returns -1 for any