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.

tests/integration/test_features.py

@@ -2275,6 +2275,36 @@ class TestPartialDir:
         partial = os.path.join(dest, ".partial", os.path.relpath(source_file, os.path.sep))
         assert not os.path.exists(partial)
 
+    def test_partial_dir_alone_implies_partial(self, shared_server):
+        """--partial-dir=DIR with no --partial implies --partial, like rsync.
+
+        rsync 3.4.1 retains the staged partial when --partial-dir is given by
+        itself; before the implication was added FastSync discarded it.  The
+        transfer is made to fail deterministically by placing a non-empty
+        directory at the destination path, so the final partial-dir ->
+        destination rename fails and whatever was staged under the partial dir
+        stays on disk."""
+        source = os.path.join(TEST_DATA_DIR, "partial_dir_implied_src")
+        dest = os.path.join(TEST_DATA_DIR, "partial_dir_implied_dst")
+        clean_dir(source)
+        clean_dir(dest)
+        source_file = os.path.join(source, "f.bin")
+        with open(source_file, "wb") as f:
+            f.write(b"partial payload")
+
+        received = get_dest_received_dir(dest, source)
+        os.makedirs(os.path.join(received, "f.bin"))
+        with open(os.path.join(received, "f.bin", "keep"), "wb") as f:
+            f.write(b"keep")
+
+        result, _ = run_client(source, dest, flags=["--partial-dir=.partial"],
+                               port=shared_server.port)
+        assert result.returncode != 0, "expected the blocked install to fail"
+
+        partial = os.path.join(dest, ".partial", os.path.relpath(source_file, os.path.sep))
+        assert os.path.exists(partial), \
+            "--partial-dir alone must imply --partial and retain the partial file"
+
 
 class TestLargeFile:
     def test_transfer_100mb_file(self, shared_server):

tests/test_client_cli.c

@@ -511,6 +511,61 @@ static void test_parse_args_ignore_existing() {
   config_delete(cfg);
 }
 
+/* --partial-dir=DIR implies --partial, matching rsync 3.4.1.  rsync resolves
+ * this after option parsing, so the implication wins over an explicit
+ * --no-partial in either order.  It is skipped under --inplace, where partial
+ * staging is bypassed and the destination is written in place. */
+static void test_parse_args_partial_dir_implies_partial() {
+  {
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", "--partial-dir=.partial", "/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->partial);
+    config_delete(cfg);
+  }
+  {
+    /* Explicit --no-partial before --partial-dir: --partial-dir still wins. */
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", "--no-partial", "--partial-dir=.partial", "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
+    EXPECT_EQ_INT(parse_args(cfg, 5, argv, positional_args, &positional_count), 0);
+    EXPECT_TRUE(cfg->partial);
+    config_delete(cfg);
+  }
+  {
+    /* Reversed order must not change the precedence. */
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", "--partial-dir=.partial", "--no-partial", "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
+    EXPECT_EQ_INT(parse_args(cfg, 5, argv, positional_args, &positional_count), 0);
+    EXPECT_TRUE(cfg->partial);
+    config_delete(cfg);
+  }
+  {
+    /* --inplace bypasses partial staging: the implication must not fire. */
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", "--inplace", "--partial-dir=.partial", "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
+    EXPECT_EQ_INT(parse_args(cfg, 5, argv, positional_args, &positional_count), 0);
+    EXPECT_FALSE(cfg->partial);
+    config_delete(cfg);
+  }
+  {
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", "--no-partial", "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
+    EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+    EXPECT_FALSE(cfg->partial);
+    config_delete(cfg);
+  }
+}
+
 static void test_parse_args_executability() {
   Config* cfg = config_create();
   char* argv[] = {"fastsync", "-E", "/src", "/dst"};
@@ -4784,6 +4839,7 @@ void test_client_cli() {
   test_parse_args_valid_port();
   test_parse_args_size_only();
   test_parse_args_ignore_existing();
+  test_parse_args_partial_dir_implies_partial();
   test_parse_args_executability();
   test_parse_args_chmod();
   test_parse_args_numeric_chmod();