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/checksum.h

@@ -9,10 +9,17 @@
  * seeded with --checksum-seed.  The ids are the values actually placed on the
  * wire (config frame), so they must be kept stable and validated on receive.
  * CHECKSUM_ALGO_XXH64 == 0 is the default and is byte-for-byte what FastSync
- * computed before these options existed (xxHash64 with seed 0). */
-typedef enum { CHECKSUM_ALGO_XXH64 = 0, CHECKSUM_ALGO_MD5 = 1 } ChecksumAlgo;
+ * computed before these options existed (xxHash64 with seed 0).  The set mirrors
+ * the algorithms rsync 3.4.1 can be built with; the ones FastSync does not
+ * implement (md4, sha1, none) are rejected by name at parse time. */
+typedef enum {
+  CHECKSUM_ALGO_XXH64 = 0,
+  CHECKSUM_ALGO_MD5 = 1,
+  CHECKSUM_ALGO_XXH3 = 2,
+  CHECKSUM_ALGO_XXH128 = 3
+} ChecksumAlgo;
 
-/* md5 digest is 16 bytes, the longest supported. */
+/* xxh128 digest is 16 bytes, the longest supported. */
 #define CHECKSUM_MAX_DIGEST_LEN 16
 
 /* Compute the whole-file digest of the first `size` bytes of `data`.
@@ -29,8 +36,10 @@ bool checksum_digest(ChecksumAlgo algo, uint64_t seed, const void* data, size_t
                      size_t out_capacity, size_t* out_len);
 
 /* Resolve a --checksum-choice string (case-insensitive) to an algorithm id.
- * Accepts "xxh64" and "xxhash" (both map to CHECKSUM_ALGO_XXH64, rsync's
- * xxhash spelling) and "md5".  Returns -1 for any unsupported name. */
+ * Accepts "xxh64"/"xxhash", "xxh3", "xxh128" and "md5".  "auto", rsync's
+ * default automatic choice, is resolved to the default by the caller (it is not
+ * a distinct algorithm here).  Returns -1 for any name FastSync does not
+ * implement (md4/sha1/none included). */
 int checksum_algo_from_name(const char* name);
 
 /* Canonical name of an algorithm (used in CLI error messages). */
@@ -39,7 +48,7 @@ const char* checksum_algo_name(ChecksumAlgo algo);
 /* True when `algo` is a supported id (used by config receive validation). */
 bool checksum_algo_valid(int algo);
 
-/* Digest length in bytes for an algorithm (xxx64 = 8, md5 = 16). */
+/* Digest length in bytes for an algorithm (xxh64/xxh3 = 8, md5/xxh128 = 16). */
 uint8_t checksum_digest_len(ChecksumAlgo algo);
 
 #endif /* CHECKSUM_H */