feat(dry-run): server-contacting --dry-run (protocol 2.21.0)

--dry-run now handshakes with a remote/daemon receiver and reports what
WOULD transfer/skip based on receiver state, mutating nothing on either
side.

- Serialize Config.dry_run into the wire config frame and append
  STATUS_DRY_RUN_TRANSFER to the status enum (no renumbering); bump
  PROTOCOL_VERSION/CMake VERSION/CHANGELOG/golden wire to 2.21.0.
- Receiver: receive_incremental_check_ex runs the normal read-only
  decision and answers STATUS_OK (skip) or STATUS_DRY_RUN_TRANSFER
  (would transfer) with no basis materialization/append/delta/full
  transfer.  All mutation sites are guarded by !dry_run: file store,
  manifest deletes, --mkpath root creation, --delay-updates staging,
  publication, directory-time application, and outcome acks.
- Client: send_dry_run_remote connects, sends the config, checks each
  regular file and prints the would-transfer set + trailer; no file data
  or delete manifest is sent.  Plain local destinations keep the
  client-side manifest.

tests/test_config.c

@@ -2347,6 +2347,7 @@ static void golden_config_populate(Config* c) {
   c->compression_level = 7;
   c->chunk_size = 65536;
   c->use_sendfile = false;
+  c->dry_run = true;
   c->use_delete = true;
   c->use_incremental = true;
   c->size_only = false;
@@ -2443,11 +2444,12 @@ static void golden_config_populate(Config* c) {
   c->copy_as_gid = 222;
 }
 
-/* The pinned golden frame (protocol 2.20.0).  The values below are the only
+/* The pinned golden frame (protocol 2.21.0).  The values below are the only
  * thing that ties the generated table to the historical wire format; update
- * them ONLY with a PROTOCOL_VERSION bump and a documented reason. */
-#define GOLDEN_WIRE_LEN 633
-#define GOLDEN_WIRE_HASH 9160991280011164139ULL
+ * them ONLY with a PROTOCOL_VERSION bump and a documented reason.  The 2.21.0
+ * bump appends the serialized dry_run bool to CONFIG_WIRE_CORE_FIELDS. */
+#define GOLDEN_WIRE_LEN 637
+#define GOLDEN_WIRE_HASH 13228626061067899189ULL
 
 static unsigned long long fnv1a_64(const unsigned char* buf, size_t len) {
   unsigned long long h = 1469598103934665603ULL;