feat(config): add fuzzy flag and bump protocol to 2.9.0

-y/--fuzzy is receiver-side similar-file basis selection, so the receiver
must learn the flag: add a bool to Config, serialize it as a trailing field
on the config frame, and validate the received value.  Because the config
frame layout changed, PROTOCOL_VERSION moves to 2.9.0 (peers must match).

src/shared/config.h

@@ -63,6 +63,14 @@ typedef struct Config {
   bool size_only;
   bool use_delta;
   bool whole_file;
+  /* -y/--fuzzy: when a file must be transferred and the destination holds no
+   * usable file at the exact path, the receiver may reuse a SIMILAR-named
+   * existing regular file in the same destination directory as the delta
+   * basis so the sender transmits only the differences.  Crosses the wire
+   * (the receiver performs the candidate search); the CLI implies
+   * --incremental + --delta because the similar-basis only matters on the
+   * receiver-driven delta path.  Off by default. */
+  bool fuzzy;
   int modify_window;
   uint32_t delta_block_size;
   unsigned long long delta_max_file_size;
@@ -206,7 +214,7 @@ typedef struct Config {
   DelayUpdatesContext* delay_context;
 } Config;
 
-#define PROTOCOL_VERSION "2.8.0"
+#define PROTOCOL_VERSION "2.9.0"
 #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