Merge feat/p2-temp-dir: implement --temp-dir

Receiver writes temps into a confined scratch dir and atomically renames
into place; EXDEV aborts; inplace/partial bypass; thread-safe temp names.
Uses existing wire field; no protocol bump. Reviewed (c-review APPROVE
WITH NITS, all fixed); PR #262.

src/client/client_cli.c

@@ -405,6 +405,7 @@ static const OptionEntry OPTION_TABLE[] = {
     {"--ca", NULL, OPT_STRING, offsetof(Config, tls_ca)},
     {"--backup-dir", NULL, OPT_STRING, offsetof(Config, backup_dir)},
     {"--fastsync-server-path", NULL, OPT_STRING, offsetof(Config, fastsync_server_path)},
+    {"--temp-dir", NULL, OPT_STRING, offsetof(Config, temp_dir)},
     {"--partial-dir", NULL, OPT_STRING, offsetof(Config, partial_dir)},
     {"--suffix", NULL, OPT_STRING, offsetof(Config, suffix)},
     {"--compress-choice", "--zc", OPT_STRING, offsetof(Config, compress_choice)},

src/client/usage.c

@@ -88,6 +88,7 @@ void print_usage(void) {
   printf("  --stderr=MODE       Route logging to stderr: errors or all\n");
   printf("  --partial           Keep partial files on interrupted transfer\n");
   printf("  --partial-dir <dir> Directory for partial files\n");
+  printf("  --temp-dir <dir>    Scratch dir for temp files before atomic install\n");
   printf("  --fastsync-server-path <path>\n");
   printf("                      Path to fastsync-server on remote (default: fastsync-server)\n");
   printf(