test: --fuzzy coverage (CLI, config wire, integration)

Unit: parse -y/--fuzzy and --no-fuzzy negation (order-independent), -W and
--no-delta leave fuzzy inert, --fuzzy implies --incremental/--delta, and
validate_config rejects fuzzy with -s (chunk serialization) and -f
(sendfile).  Config: fuzzy survives the socketpair wire round-trip.

Integration (TestFuzzy, byte counts via a new CountingProxy helper): the
rename case transfers a 2 MiB file in a few percent of its size with byte-
exact output under --fuzzy, while the no-fuzzy, no-candidate, dissimilar-
sibling and --whole-file runs send the whole file; -y alias; -m parity;
dest-holds-unsuitable-file falls back to the sibling; --delay-updates and
--remove-source-files keep their semantics on fuzzy transfers.

tests/test_config.c

@@ -128,6 +128,7 @@ static void test_config_send_receive() {
   send_cfg->use_executability = true;
   send_cfg->use_delta = true;
   send_cfg->whole_file = true;
+  send_cfg->fuzzy = true;
   send_cfg->ignore_times = true;
   send_cfg->size_only = true;
   send_cfg->compression_level = 5;
@@ -188,6 +189,8 @@ static void test_config_send_receive() {
         ok = false;
       if (recv_cfg->use_delta)
         ok = false;
+      if (!recv_cfg->fuzzy)
+        ok = false;
       if (recv_cfg->modify_window != 4)
         ok = false;
       if (!recv_cfg->existing)