refactor(dir-times): extract dir_times_should_capture predicate

Deduplicate the repeated directory-time capture gate
(`config->use_metadata && !config->omit_dir_times`) used by the
sender-side (multiprocessing.c) and receiver-side (receiver.c) sinks
into a single predicate declared next to the DirTimeList machinery in
file_receive.h and defined in file_receive.c.

Behavior preserved: identical short-circuit condition and semantics,
no signature or protocol changes.

src/shared/file_receive.h

@@ -30,6 +30,12 @@ typedef struct {
   size_t capacity;
 } DirTimeList;
 
+/* Capture gate shared by the sender-side and receiver-side sinks: directory
+ * metadata is accumulated only when --times/--metadata is in effect and
+ * -O/--omit-dir-times does not suppress it.  Kept here, next to the accumulator
+ * it guards, so both call sites express the same condition. */
+bool dir_times_should_capture(const Config* config);
+
 void dir_time_list_init(DirTimeList* list);
 void dir_time_list_free(DirTimeList* list);
 /* Deep-copy one directory's path + metadata into the list.  Returns false on