preallocate: --preallocate allocates dest space up front

Receiver allocates the destination file's full size before streaming data
(posix_fallocate preferred, ftruncate fallback on EOPNOTSUPP/ENOSYS) so an
out-of-space transfer fails fast instead of partway. Additive config bool
crossing the wire; PROTOCOL_VERSION 2.10.0 -> 2.11.0. Threaded through all
store paths (atomic, inplace, partial/delay-updates staging, link-dest copy
fallback). Review hardening: explicit lseek(0) before the data write so
correctness does not depend on posix_fallocate leaving the fd offset unchanged.

tests/test_client_cli.c

@@ -2083,8 +2083,24 @@ static void test_validate_config_append_verify_rejects_whole_file() {
   EXPECT_FALSE(validate_config(cfg));
   config_delete(cfg);
 }
+
+/* --preallocate parses as a boolean flag and validates cleanly. */
+static void test_parse_args_preallocate() {
+  Config* cfg = config_create();
+  cfg->send_directory = str_dup("/src");
+  cfg->receive_root_directory = str_dup("/dst");
+  char* argv[] = {"fastsync", "--preallocate", "/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->preallocate);
+  EXPECT_TRUE(validate_config(cfg));
+  config_delete(cfg);
+}
+
 void test_client_cli() {
   test_validate_config_required_paths();
+  test_parse_args_preallocate();
   test_parse_args_append();
   test_parse_args_append_verify();
   test_parse_args_append_both();