Live browser view while scanning, with downloads and screenshots

scan.py --view serves a page (myscan/webview.py, standard library only) with
MJPEG streams of the latest camera frame and the page as it is stitched,
Start/Stop buttons, PNG/JPEG download of the finished page and a screenshot
of the current view.

myscan/live.py keeps a downscaled preview: new frames are pasted at their
current best pose, and a background thread re-solves the pose graph every
1.5 s (throttled to at most ~50% duty) and redraws the preview off the
registration thread. PageGraph.solve can now run on a prefix of the frames
while add() continues, keeping loop closures and features between solves,
so the final solve after the stop press is much shorter. Frame PNGs are
saved on their own thread.

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

README.md

@@ -9,7 +9,11 @@ Linux driver tools for the Hama myScan (00052343) scanner mouse, a Dacuda design
 - `myscan/pagegraph.py` — assemble a page from all frames at once: frame-to-frame registration, loop closures
   between passes, pose-graph optimisation (used by `scan.py` and `stitch.py`)
 - `myscan/stitch.py` — the older live, frame-by-frame stitcher (`stitch.py --live`) and shared helpers
-- `tools/scan.py` — scan with the button: `python tools/scan.py` (press to start, press again to stop)
+- `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
+  (re-solved every 1.5 s, current position outlined), Start/Stop buttons, PNG/JPEG download of the finished
+  page and a Screenshot button (camera + page + status as one PNG)
+- `myscan/live.py`, `myscan/webview.py` — the live preview and its browser viewer (standard library only)
 - `tools/record.py` — record a scan session: `python tools/record.py -d 25 -o scan_text`
 - `tools/stitch.py` — stitch a recording: `python tools/stitch.py scan_text` -> `scan_text/page.png`
 - `tools/probe.py` — log raw frames and HID reports side by side