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/client/change_list.c

@@ -158,14 +158,6 @@ static void itemize_code(const Config* config, const ChangeEvent* event, char co
   code[11] = '\0';
 }
 
-char* change_render_itemize_code(const Config* config, const ChangeEvent* event) {
-  if (event == NULL || event->decision != CHANGE_SENT)
-    return str_dup("");
-  char code[12];
-  itemize_code(config, event, code);
-  return str_dup(code);
-}
-
 /* rsync %n: the transfer-relative name, with a trailing slash for directories. */
 static bool append_name(StrBuf* buf, const ChangeEvent* event) {
   if (!strbuf_append(buf, event->name != NULL ? event->name : ""))

src/client/change_list.h

@@ -63,9 +63,6 @@ bool change_list_enabled(const Config* config);
  * (`%i %n%L`): `>f+++++++++ sub/b.txt`.  Caller frees the result. */
 char* change_render_itemize(const Config* config, const ChangeEvent* event);
 
-/* Render only the 11-character itemize code (rsync %i).  Caller frees. */
-char* change_render_itemize_code(const Config* config, const ChangeEvent* event);
-
 /* Expand an --out-format/--log-file-format template.  Supported tokens:
  *   %i  itemize code            %n  transfer-relative name (dir: trailing /)
  *   %f  long display path       %l  file length in bytes

src/client/client_send.c

@@ -1666,6 +1666,10 @@ static int send_delta(Client* client, File* file, DeltaSignature* sig, Config* c
 static int send_append(const Client* client, File* file, Config* config,
                        unsigned long long offset) {
   int fd = client->file_descriptor;
+  if (file->data->data == NULL && !file_load_data(file)) {
+    send_status(fd, STATUS_ERROR);
+    return -1;
+  }
   const unsigned long long fsize = file->data->size;
   if (offset >= fsize) {
     send_status(fd, STATUS_ERROR);