fix(display): only mtime-touch a snapshot frame that is actually on disk

From review (CodeRabbit): the digest was recorded when a frame was queued,
so while the writer hadn't saved it yet, an unchanged frame could take
the TOUCH path and refresh the mtime of the older file on disk. Track the
saved frame's digest separately (set by the writer and the inline path
after a successful save) and write, not touch, when it isn't the current
frame.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_display_dirty_tracking.py

@@ -425,6 +425,32 @@ class TestSnapshotOffRenderThread:
             dm.set_scrolling_state(False)
         assert dm._last_snapshot_digest is None
 
+    def test_a_frame_not_yet_on_disk_is_written_not_touched(
+            self, dm, tmp_path, monkeypatch):
+        # The digest is recorded when a frame is queued. Until the writer has
+        # saved it, an unchanged frame must not mtime-touch the older file on
+        # disk into looking current.
+        import zlib
+        from src.common import snapshot_policy
+        touched, saved = [], []
+        self._due(dm, tmp_path, (9, 9, 9))
+        digest = zlib.adler32(dm.image.tobytes())
+        dm._last_snapshot_digest = digest        # queued earlier...
+        dm._saved_snapshot_digest = 12345        # ...but an older frame is on disk
+        monkeypatch.setattr(snapshot_policy, "decide",
+                            lambda *a, **k: snapshot_policy.SnapshotAction.TOUCH)
+        monkeypatch.setattr(os, "utime", lambda *a, **k: touched.append(a))
+        monkeypatch.setattr(dm, "_save_snapshot", lambda image: saved.append(image))
+        dm.set_scrolling_state(False)
+        dm._write_snapshot_if_due(digest)
+        assert touched == []
+        assert len(saved) == 1
+        assert dm._saved_snapshot_digest == digest
+
+        # Once it is on disk, the same frame is only touched.
+        dm._write_snapshot_if_due(digest)
+        assert len(touched) == 1 and len(saved) == 1
+
     def test_a_static_frame_lands_after_a_queued_one_still_being_written(
             self, dm, tmp_path, monkeypatch):
         # The last frame of a scroll can still be encoding when the first