AR: sub-pixel corner refinement, ROI-first detection, anchor coplanarity check

README.md

@@ -51,6 +51,20 @@ Dev HUD overlay tap zones (thirds):
 Engine choice persists in `localStorage` (`nbx.engine`). Optional per-device
 horizontal FOV override: `localStorage nbx.hfovDeg` (default 60, both engines).
 
+`localStorage` knobs:
+
+| key              | default | what it does |
+|------------------|---------|--------------|
+| `nbx.engine`     | opencv  | `opencv` board solve, or `posit` (v2 fallback) |
+| `nbx.hfovDeg`    | 60      | assumed horizontal FOV, applied to the image's long axis |
+| `nbx.detectHz`   | 20      | detection rate; rendering stays at full frame rate |
+| `nbx.detectLong` | 960     | long side of the full-scan detection canvas |
+| `nbx.subpix`     | 1       | sub-pixel corner refinement; `0` disables |
+| `nbx.roi`        | 1       | ROI-first detection; `0` forces a full scan every tick |
+
+Set `nbx.subpix=0` and `nbx.roi=0` to get exactly the pre-October detection
+behaviour, which is the A/B to run if tracking regresses.
+
 `axis ymirror` exists as on-device insurance: if the world appears mirrored /
 camera under the table in opencv mode, tap the middle third once.