feat(parity): rsync 3.4.1 checksum/timeout/temp-dir/connectivity parity (#289 #295 #296)

#289 checksum/compression:
- -c/--checksum now implies the incremental content quick-check (without
  implying -t), so an unchanged file is skipped like rsync.
- --checksum-choice/--cc accepts xxh64/xxhash, xxh3, xxh128, md5 and auto;
  md4/sha1/none and the two-name form are rejected by name.
- --compress-choice/--zc rejects lz4/zlib/zlibx by name (zstd/none/auto kept).
- --checksum-seed=0 is randomized per transfer and sent on the wire.
- --skip-compress uses rsync 3.4.1's default suffix list; slash separators and
  dot-less suffixes are accepted.
- add --no-whole-file.

#295 timeouts/alloc/temp-dir:
- --timeout default 0 (disabled), --contimeout default 60; 0 disables both,
  plus --no-timeout/--no-contimeout.
- --max-alloc=0 means no allocation limit (was rejected).
- --temp-dir accepts any dir, requires it to exist, and falls back to a
  non-atomic copy on EXDEV instead of aborting.

#296 connectivity/daemon:
- -M/--remote-option is rejected for daemon/TCP destinations (SSH-only).
- --trust-sender clarified as receiver-local; server-path tests added.
- --stop-at accepts rsync's full date form (y-m-dTh:m etc.).

Adds unit and integration coverage; no wire-field change, PROTOCOL_VERSION stays
2.22.0.

src/shared/file_send.c

@@ -143,20 +143,28 @@ bool file_send_sendfile_with_skip(File* file, int file_descriptor, bool use_meta
   }
 
   off_t offset = 0;
+  /* A non-positive --timeout disables the deadline: poll blocks until the
+   * socket is writable (rsync's --timeout=0 default). */
+  int io_timeout_sec = protocol_get_io_timeout_sec();
   struct timespec deadline;
-  clock_gettime(CLOCK_MONOTONIC, &deadline);
-  deadline.tv_sec += protocol_get_io_timeout_sec();
+  if (io_timeout_sec > 0) {
+    clock_gettime(CLOCK_MONOTONIC, &deadline);
+    deadline.tv_sec += io_timeout_sec;
+  }
   while ((unsigned long long)offset < file_size) {
-    struct timespec now;
-    clock_gettime(CLOCK_MONOTONIC, &now);
-    long long remaining = (long long)(deadline.tv_sec - now.tv_sec) * 1000LL +
-                          (deadline.tv_nsec - now.tv_nsec) / 1000000LL;
-    if (remaining <= 0) {
-      close(fd);
-      return false;
+    int timeout = -1;
+    if (io_timeout_sec > 0) {
+      struct timespec now;
+      clock_gettime(CLOCK_MONOTONIC, &now);
+      long long remaining = (long long)(deadline.tv_sec - now.tv_sec) * 1000LL +
+                            (deadline.tv_nsec - now.tv_nsec) / 1000000LL;
+      if (remaining <= 0) {
+        close(fd);
+        return false;
+      }
+      timeout = remaining > INT_MAX ? INT_MAX : (int)remaining;
     }
     struct pollfd pfd = {.fd = file_descriptor, .events = POLLOUT};
-    int timeout = remaining > INT_MAX ? INT_MAX : (int)remaining;
     int polled = poll(&pfd, 1, timeout);
     if (polled <= 0 || (pfd.revents & (POLLERR | POLLHUP | POLLNVAL))) {
       close(fd);