metadata times: -U/--atimes, -N/--crtimes, --open-noatime, -O/-J

Capture+transmit source atime (pre-read stat; O_NOATIME sender guard) and birth
time (statx STATX_BTIME); receiver restores atime with mtime (crtime not settable
portably -> transmitted, explicitly not applied). --open-noatime is client-only.
-O/-J documented as accepted no-ops (FastSync never preserves dir/symlink times).
Wire: metadata frame gains atime/crtime val+sec+nsec; PROTOCOL_VERSION
2.11.0->2.12.0. Review fixes: gate atime capture to Linux (no epoch clobber on
non-Linux), close fd on fdopen failure, honest -O/-J status (Compat no-op).

src/client/scanner.h

@@ -14,6 +14,10 @@
 
 typedef struct {
   bool use_metadata;
+  /* Phase 4 metadata capture: -U/--atimes and -N/--crtimes tell the scanner to
+   * capture the source access / birth time into each entry's FileMetadata. */
+  bool preserve_atimes;
+  bool preserve_crtimes;
   unsigned long long chunk_size;
   char** exclude_patterns;
   int exclude_count;
@@ -75,6 +79,8 @@ typedef struct {
   DIR* current_dir;
   char* current_path;
   bool use_metadata;
+  bool preserve_atimes;
+  bool preserve_crtimes;
   unsigned long long chunk_size;
   char** exclude_patterns;
   int exclude_count;