perf(display): skip preview work nobody reads (lazy checksum, 1 Hz snapshot writer) (#717)

Mid-scroll, update_display() no longer checksums every frame: it asks is_currently_scrolling() once per frame and reuses the answer, hashes only when dirty tracking can skip a static frame, and the preview snapshot asks its policy first and hashes only when a write or touch could follow (decide() is monotone, pinned by a property test). With the preview open the snapshot is written at most once a second (VIEWER_INTERVAL 1.0 s, was 0.2 s; the SSE stream re-read it once a second, so four encodes in five went unread); the stream now polls its mtime every 0.25 s (VIEWER_POLL_INTERVAL), so the preview stays about as fresh. The PNG is written at compress_level=1. --preview soaks are not comparable across this change.

Merged with #716: _scan_segments takes the frame's one scrolling answer and #716's static-handover pass-through, as soaked on ledpi (A B B A, 20 min each: main 0.118% / 0.113% late, with #716 and this 0.107% / 0.104%).

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

scripts/frame_soak.py

@@ -9,7 +9,10 @@ reports the difference. Nothing is stopped, restarted or drawn.
     python3 scripts/frame_soak.py
 
     # the same with the web preview open (the preview's PNG encodes are one of
-    # the things that used to make the render loop miss refreshes)
+    # the things that used to make the render loop miss refreshes). An open
+    # preview is encoded at most once a second; through 3.8.0 it was up to
+    # five times, so a --preview run from before that change is not comparable
+    # with one from after it
     python3 scripts/frame_soak.py --preview
 
     # quick look at the totals since the service started
@@ -65,7 +68,8 @@ from src.common.frame_timing import (  # noqa: E402
 )
 
 #: Touched by the web UI while someone has the preview open; a fresh marker
-#: puts the display service's snapshot writer at full rate. Same path as
+#: puts the display service's snapshot writer at the viewer rate
+#: (snapshot_policy.VIEWER_INTERVAL). Same path as
 #: DisplayManager._viewer_marker_path.
 VIEWER_MARKER = "/tmp/led_matrix_preview_viewer"  # nosec B108 - fixed path shared with the service