enhancer: test stale pre-existing destination export (ku0)

The _save_buffer fix that rejects pre-existence as proof of a
successful save (mtime/size change verification) was already in place.
Add the missing test case (c) for a stale pre-existing regular file at
the destination, which the task requires alongside the directory and
unwritable-destination cases:

  /enhancer/export_stale_dest:
   (a) writable stale file -> correctly overwritten; assert the new PNG
       bytes differ from the stale content (not mere presence).
   (b) read-only stale file the saver cannot replace -> export returns
       FALSE with a GError; skipped under root (root bypasses file-mode
       perms; the directory case in export_real_success covers a
       root-safe save failure).

Gated behind gegl_dep.found() (same as the rest of test_enhancer), so
the minimal (gegl=disabled) lane is unaffected. All 7 enhancer subtests
+ the full 24-test suite pass; ASan/LSan clean.

0 files changed, 0 insertions, 0 deletions