feat(checksum): --checksum-choice/--cc and --checksum-seed for whole-file digest

Adds real algorithm selection (xxh64 default, plus md5 via OpenSSL EVP) and a
64-bit seed for the per-file whole-file digest used by the --incremental/
--checksum handshake and basis-dir content verification. The seed also feeds
the delta path's per-block xxHash32 strong checksum (low 32 bits) so an
explicit seed deterministically changes those digests too. Sender and receiver
hash identically: the algorithm id and seed cross the config wire frame and the
STATUS_CHECK handshake now carries a length-prefixed, bounded digest instead of
a fixed 64-bit value. Unsupported algorithm names are rejected at parse time
(never a silent no-op). PROTOCOL_VERSION bumped 2.9.0 -> 2.10.0; defaults
(xxh64, seed 0) preserve prior byte-for-byte behavior.

src/shared/config.h

@@ -2,6 +2,7 @@
 #define CONFIG_H
 
 #include "array_list.h"
+#include "checksum.h"
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -222,6 +223,18 @@ typedef struct Config {
   char* compress_choice;
   char* chmod_spec;
 
+  /* --checksum-choice / --cc and --checksum-seed.  checksum_algo is the id of
+   * the whole-file content-digest algorithm used by the per-file --incremental
+   * handshake (sender computes it, receiver compares it to skip unchanged
+   * files) and by the basis-dir content verification.  checksum_seed is passed
+   * to xxHash64 (and to the delta block strong hash, low 32 bits); md5 has no
+   * seed so it is ignored there.  Both cross the wire: the receiver MUST hash
+   * the on-disk old file with the same algorithm and seed to reach a matching
+   * digest.  Defaults (XXH64 / seed 0) reproduce the pre-existing behavior
+   * byte-for-byte. */
+  int checksum_algo;      /* ChecksumAlgo, default CHECKSUM_ALGO_XXH64 */
+  uint64_t checksum_seed; /* default 0 */
+
   char** skip_compress_suffixes;
   int skip_compress_count;
   bool skip_compress_set;
@@ -231,7 +244,7 @@ typedef struct Config {
   DelayUpdatesContext* delay_context;
 } Config;
 
-#define PROTOCOL_VERSION "2.9.0"
+#define PROTOCOL_VERSION "2.10.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
 /* Upper bound on total basis-dir entries (rsync caps --link-dest at 20). */
 #define MAX_BASIS_DIRS 64