feat(basis): rsync quick-check default + FastSync-only --verify-basis

- default basis match is rsync's metadata quick-check (size + mtime; size-only
  drops mtime; -I disables), no mandatory content digest
- new long-only --verify-basis (wire bool, protocol stays 2.28.0) restores the
  strict whole-file content equality
- --copy-dest re-applies source attributes; basis-hit 256 MiB cap removed by
  streaming the copy/hash; basis miss keeps the normal payload bound
- compare/copy/link-dest rows -> caveat; tally 116/13/28

tests/test_xattr.c

@@ -210,7 +210,8 @@ static void test_xattr_receive_drops_acl_without_preserve_acls() {
 
 /* MINOR-2: a --link-dest / -H copy fallback (linkat refused) must still apply
  * the per-file xattrs and --fake-super stat.  A DIRECTORY basis forces linkat
- * to fail with EPERM, exercising the byte-copy fallback deterministically.
+ * to fail with EPERM, exercising the byte-copy fallback deterministically (the
+ * basis is not a regular file, so the fallback uses the caller's bytes).
  * Guarded on filesystem xattr support. */
 static void test_link_copy_fallback_preserves_xattrs() {
   const char* dest = "test_link_xattr_dest.txt";