feat: implement --delay-updates receiver staging and publication

Stage every successfully written file under a private 0700 .fastsync-stage
directory inside the receive root and atomically publish all staged files
only after the whole protocol stream (manifest/delete handling included)
has completed, immediately before the success/outcome frame.  On any
abort/error before publication nothing is installed and staging is removed;
a publish failure aborts the transfer with best-effort cleanup of the
remainder (already-published files are not rolled back).  Crash leftovers
are wiped when the next delayed transfer starts.

Wire: new delay_updates config flag (selection-options block), protocol
version bumped to 2.6.0, client/server validation rejects --inplace.
CLI/usage/validation updated.  Works in single-threaded and -m modes
(exactly one write_thread stages files; the staged-file registry is
mutex-protected; publication runs once after both threads join).
--existing/--ignore-existing/--update decide against the final destination
at stage time; --backup is deferred to publication.  remove_source_files
outcomes are only sent after publication so skipped/unpublished sources are
never deleted.  Default (no flag) behavior is unchanged.

Tests: config wire round-trip, CLI parse, --inplace rejection, new
test_delay_updates unit suite (27 suites total), and integration
TestDelayUpdates covering single/-m parity, incremental reruns, remove
source files, receiver-skip ordering, and a deterministic publish-failure
abort path.

tests/test_client_cli.c

@@ -1265,6 +1265,28 @@ static void test_parse_args_log_file_format() {
   config_delete(cfg);
 }
 
+/* --delay-updates is a plain boolean receiver option. */
+static void test_parse_args_delay_updates() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--delay-updates", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+  EXPECT_TRUE(cfg->delay_updates);
+  EXPECT_EQ_INT(positional_count, 2);
+  config_delete(cfg);
+}
+
+/* rsync rejects --delay-updates with --inplace; FastSync must too. */
+static void test_validate_config_delay_updates_rejects_inplace() {
+  Config* cfg = valid_client_config();
+  cfg->delay_updates = true;
+  cfg->inplace = true;
+  EXPECT_FALSE(validate_config(cfg));
+  config_delete(cfg);
+}
+
 void test_client_cli() {
   test_validate_config_required_paths();
   test_validate_config_incompatible_options();
@@ -1344,4 +1366,6 @@ void test_client_cli() {
   test_parse_args_checksum_choice_aliases();
   test_parse_args_checksum_choice_requires_value();
   test_parse_args_temp_dir();
+  test_parse_args_delay_updates();
+  test_validate_config_delay_updates_rejects_inplace();
 }