fix(cli): --partial-dir implies --partial (rsync parity)

rsync 3.4.1 resolves --partial-dir after option parsing and sets keep_partial,
so --partial-dir=DIR alone retains an interrupted transfer's partial file.
FastSync only used the partial dir when --partial was also given, silently
discarding it otherwise.

Set Config->partial in cli_finalize_config whenever partial_dir is set.
Following rsync, an explicit --no-partial does NOT win (verified on rsync
3.4.1 in either option order); --inplace is guarded because it writes the
destination in place with no partial staging.

Tests: CLI unit coverage for the implication/precedence/inplace guard, and a
deterministic integration case that blocks the final install (non-empty
directory at the destination) and asserts the staged partial survives under
--partial-dir alone.

src/client/client_cli.c

@@ -2612,6 +2612,15 @@ static int cli_finalize_config(Config* config, bool verbose, bool no_delta, bool
   if (config->use_delete && !config->delete_before && !config->delete_during &&
       !config->delete_delay && !config->delete_after)
     config->delete_during = true;
+  /* rsync parity: --partial-dir=DIR chooses where an interrupted transfer's
+     partial file is kept, so it implies --partial.  rsync applies the
+     implication after option parsing, so it wins over an explicit --no-partial
+     regardless of the order the two options appear in (verified on rsync
+     3.4.1).  --inplace is the exception: the destination file is written in
+     place with no partial/temp staging, so the partial machinery is bypassed
+     and the implication is skipped to leave --inplace behavior untouched. */
+  if (config->partial_dir && !config->inplace)
+    config->partial = true;
   if (config->compress_choice) {
     int algo = compression_algo_from_name(config->compress_choice);
     if (algo >= 0) {

src/client/usage.c

@@ -288,7 +288,7 @@ void print_usage(void) {
   printf("  --log-file <path>, --log-file=<path>  Write log messages to file\n");
   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("  --partial-dir <dir> Directory for partial files (implies --partial)\n");
   printf("  -T, --temp-dir <dir>  Scratch dir for temp files before atomic install.\n");
   printf("                      Confined to the receive root: a relative dir resolves below\n");
   printf("                      it and an absolute/traversal dir is rejected.  The dir must\n");