Drop frames taken while the mouse lies still

PageGraph.add skips a frame when neither the laser odometry nor a small
thumbnail shows it moved since the last kept frame, so resting on the page
no longer costs registration, solving and rendering time. LiveSession maps
captures to the kept frames for its sensor readings.

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

README.md

@@ -13,7 +13,8 @@ Linux driver tools for the Hama myScan (00052343) scanner mouse, a Dacuda design
   prior; another device can pass its own object with `predict()`), `register` (ECC, phase correlation, SIFT
   matching, overlap check), `canvas` (weighted canvases, page composition, parallel rendering), `posegraph`
   (robust pose-graph optimisation), `loops` (loop-closure candidates), and the two stitchers built from them:
-  `pagegraph` (`PageGraph`: assembles the page from all frames at once with loop closures between passes;
+  `pagegraph` (`PageGraph`: assembles the page from all frames at once with loop closures between passes,
+  dropping frames taken while the mouse lies still;
   used by `scan.py` and `stitch.py`) and `incremental` (`Stitcher`: frame by frame, `stitch.py --live`)
 - `tools/scan.py` — scan with the button: `python tools/scan.py` (press to start, press again to stop).
   `--view [PORT]` serves a live view at `http://<host>:8000/`: the camera, the page as it is stitched