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/file.c

@@ -43,17 +43,17 @@ static unsigned long long next_temp_sequence(void) {
   return atomic_fetch_add_explicit(&sequence, 1, memory_order_relaxed);
 }
 
-bool file_checksum(File* file, uint64_t* checksum) {
-  if (!file || !checksum || !file->data)
+bool file_checksum(File* file, ChecksumAlgo algo, uint64_t seed, uint8_t* out, size_t out_capacity,
+                   size_t* out_len) {
+  if (!file || !out || !out_len || !file->data)
     return false;
   if (file->data->size == 0) {
-    *checksum = delta_xxhash64("", 0);
-    return true;
+    return checksum_digest(algo, seed, "", 0, out, out_capacity, out_len);
   }
   if (!file->data->data && !file_load_data(file))
     return false;
-  *checksum = delta_xxhash64(file->data->data, file->data->size);
-  return true;
+  return checksum_digest(algo, seed, file->data->data, file->data->size, out, out_capacity,
+                         out_len);
 }
 
 File* file_create(const char* path) {