feat(codec): implement md4/sha1/none digests and lz4/zlib/zlibx codecs

Add real implementations for the rsync 3.4.1 checksum and compression
breadth: a self-contained MD4 (RFC 1320), OpenSSL-backed SHA1, a
no-digest mode, and LZ4/zlib codecs alongside zstd.  Compressed buffers
are now self-describing (a leading codec id), so every existing
decompression call site keeps working through a process-global codec
selection.  zlibx shares the zlib codec because FastSync compresses only
delta/token bytes (never matched file data), matching the 'x' intent.

src/shared/checksum.h

@@ -8,25 +8,35 @@
 /* Whole-file content-digest algorithms selectable with --checksum-choice and
  * 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).  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. */
+ * CHECKSUM_ALGO_XXH64 == 0 is the historical FastSync default and its numeric
+ * value is preserved.  The full set mirrors the algorithms rsync 3.4.1 can be
+ * built with; every one of them is implemented here. */
 typedef enum {
   CHECKSUM_ALGO_XXH64 = 0,
   CHECKSUM_ALGO_MD5 = 1,
   CHECKSUM_ALGO_XXH3 = 2,
-  CHECKSUM_ALGO_XXH128 = 3
+  CHECKSUM_ALGO_XXH128 = 3,
+  CHECKSUM_ALGO_MD4 = 4,
+  CHECKSUM_ALGO_SHA1 = 5,
+  CHECKSUM_ALGO_NONE = 6
 } ChecksumAlgo;
 
-/* xxh128 digest is 16 bytes, the longest supported. */
-#define CHECKSUM_MAX_DIGEST_LEN 16
+/* FastSync's negotiated default (rsync 3.4.1 auto-negotiates xxh128 first).
+ * The wire default for Config->checksum_algo is this value. */
+#define CHECKSUM_ALGO_DEFAULT CHECKSUM_ALGO_XXH128
+
+/* sha1 digest is 20 bytes, the longest supported. */
+#define CHECKSUM_MAX_DIGEST_LEN 20
 
 /* Compute the whole-file digest of the first `size` bytes of `data`.
  *
  *  - CHECKSUM_ALGO_XXH64: xxHash64(data, size, seed) (full 64-bit seed).
- *  - CHECKSUM_ALGO_MD5:   md5(data, size) via OpenSSL EVP.
- *                          md5 has no seed, so `seed` is ignored (documented).
+ *  - CHECKSUM_ALGO_XXH3:  XXH3_64bits_withSeed(data, size, seed).
+ *  - CHECKSUM_ALGO_XXH128: XXH3_128bits_withSeed(data, size, seed).
+ *  - CHECKSUM_ALGO_MD5:   md5(data, size) via OpenSSL EVP (seed ignored).
+ *  - CHECKSUM_ALGO_MD4:   md4(data, size), self-contained RFC 1320 (seed ignored).
+ *  - CHECKSUM_ALGO_SHA1:  sha1(data, size) via OpenSSL EVP (seed ignored).
+ *  - CHECKSUM_ALGO_NONE:  no digest; *out_len is 0 and nothing is written.
  *  - `size == 0` hashes the empty input (plus its seed), not a NULL input.
  *
  * Writes up to `out_capacity` bytes into `out`, storing the digest length in
@@ -36,10 +46,9 @@ 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"/"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). */
+ * Accepts "xxh64"/"xxhash", "xxh3", "xxh128", "md5", "md4", "sha1", "none".
+ * "auto" is not an algorithm here; the caller resolves it to the negotiated
+ * default.  Returns -1 for any unrecognized name. */
 int checksum_algo_from_name(const char* name);
 
 /* Canonical name of an algorithm (used in CLI error messages). */
@@ -48,7 +57,13 @@ 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 (xxh64/xxh3 = 8, md5/xxh128 = 16). */
+/* Digest length in bytes for an algorithm (xxh64/xxh3 = 8,
+ * md5/md4/xxh128 = 16, sha1 = 20, none = 0). */
 uint8_t checksum_digest_len(ChecksumAlgo algo);
 
-#endif /* CHECKSUM_H */
+/* Pick the first algorithm from FastSync's compiled-in preference list that is
+ * supported on this build (rsync 3.4.1's `--version` order:
+ * xxh128 xxh3 xxh64 md5 md4 sha1 none).  Used to resolve "auto". */
+ChecksumAlgo checksum_negotiate_default(void);
+
+#endif /* CHECKSUM_H */