feat(cli): parse -y/--fuzzy and --no-fuzzy

Register --fuzzy (alias -y) as a boolean option and fuzzy as negatable so
--no-fuzzy works through the generic negation machinery.  FastSync's delta
machinery is off by default (unlike rsync, where --fuzzy implies nothing
because delta is the default), so --fuzzy implies --incremental and --delta
unless --whole-file or an explicit --no-delta switched delta off (leaving
fuzzy inert, matching rsync where -W makes fuzzy irrelevant).  Add help text.

src/client/client_cli.c

@@ -418,6 +418,7 @@ static const OptionEntry OPTION_TABLE[] = {
     {"--modify-window", "-@", OPT_NONNEG_INT, offsetof(Config, modify_window)},
     {"--delta", NULL, OPT_FLAG, offsetof(Config, use_delta)},
     {"--whole-file", "-W", OPT_FLAG, offsetof(Config, whole_file)},
+    {"--fuzzy", "-y", OPT_FLAG, offsetof(Config, fuzzy)},
     {"--save-to-disk", NULL, OPT_FLAG, offsetof(Config, save_to_disk)},
     {"--progress", NULL, OPT_FLAG, offsetof(Config, show_progress)},
     {"--tls", NULL, OPT_FLAG, offsetof(Config, use_tls)},
@@ -487,6 +488,7 @@ static const NegatableOption NEGATABLE_OPTIONS[] = {
     {"delete", NULL, offsetof(Config, use_delete)},
     {"incremental", NULL, offsetof(Config, use_incremental)},
     {"delta", NULL, offsetof(Config, use_delta)},
+    {"fuzzy", NULL, offsetof(Config, fuzzy)},
     {"save-to-disk", NULL, offsetof(Config, save_to_disk)},
     {"progress", NULL, offsetof(Config, show_progress)},
     {"tls", NULL, offsetof(Config, use_tls)},
@@ -608,6 +610,9 @@ static int apply_table_option(Config* config, const OptionEntry* entry, const ch
 int parse_args(Config* config, int argc, char* argv[], int* positional_args,
                int* positional_count) {
   bool verbose = false;
+  /* --no-delta seen on the command line: the user explicitly switched the
+     delta machinery off, so the --fuzzy implication must not override it. */
+  bool no_delta = false;
   protocol_set_8_bit_output(config->eight_bit_output);
 
   /* Apply output controls before processing other options so their order is irrelevant. */
@@ -637,6 +642,8 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
       continue;
     }
     if (strncmp(argv[i], "--no-", strlen("--no-")) == 0) {
+      if (strcmp(argv[i], "--no-delta") == 0)
+        no_delta = true;
       if (apply_negation(config, argv[i]) != 0)
         return -1;
       continue;
@@ -1068,6 +1075,19 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
   if (config_has_basis(config))
     config->use_incremental = true;
 
+  /* -y/--fuzzy reuses an existing similar-named destination file as the delta
+   * basis, so it is meaningless without the receiver-driven delta path:
+   * imply --incremental, and --delta unless --whole-file (or an explicit
+   * --no-delta) switched the delta machinery off.  FastSync has delta OFF by
+   * default (unlike rsync), so a bare --fuzzy must turn it on or it would be
+   * a silent no-op.  --whole-file/--no-delta after --fuzzy therefore leave
+   * fuzzy inert, matching rsync where --fuzzy only affects delta transfers. */
+  if (config->fuzzy) {
+    config->use_incremental = true;
+    if (!config->whole_file && !no_delta)
+      config->use_delta = true;
+  }
+
   /* Incremental and delta transfers need metadata unless the user disabled it. */
   if ((config->use_incremental || config->use_delta) && !config->use_metadata &&
       !config->metadata_explicitly_disabled) {

src/client/usage.c

@@ -81,6 +81,11 @@ void print_usage(void) {
          "used)\n");
   printf("  --delta             Delta transfer for changed files (requires --incremental)\n");
   printf("  -W, --whole-file    Transfer changed files without delta processing\n");
+  printf("  -y, --fuzzy         Use a similar-named file already in the destination\n");
+  printf("                    directory as the delta basis when the destination has no\n");
+  printf("                    usable file at the exact path (saves bandwidth; implies\n");
+  printf("                    --incremental and --delta; inert with --whole-file)\n");
+  printf("  --no-fuzzy          Disable --fuzzy\n");
   printf("  --delta-block <n>   Delta block size in bytes (default: %d)\n",
          DELTA_BLOCK_SIZE_DEFAULT);
   printf("  --delta-max <n>     Max file size for delta transfer (default: %llu)\n",