docs: add box+# calibration target procedure to distortion spec

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-06-11-wide-angle-distortion-design.md

@@ -19,8 +19,8 @@ This is a **model** limitation, not a calibration-points problem.
 - Camera is **permanently fixed** relative to the bed → calibration is one-time and durable.
 - Correction strategy: **bend the overlay** to match the raw (curved) video. The camera
   image is left untouched (no per-frame image rectification).
-- Calibration budget: **~8–12 points** jogged to known machine X/Y across the bed,
-  including near the corners (where distortion is worst).
+- Calibration budget: **~12–16 points** jogged to known machine X/Y across the bed,
+  including near the corners (where distortion is worst). See *Calibration target* below.
 - Keep the existing overlay pipeline shape: `machine-mm → image-point` for drawing, and
   `image-point → machine-mm` for click-to-set-origin / drag-to-rotate.
 
@@ -92,6 +92,32 @@ video click (px)
   → set work origin / rotation
 ```
 
+## Calibration target (practical procedure)
+
+A **box + "#"** drawn on the bed: lines at the thirds of each axis, i.e.
+`x ∈ {0, ⅓, ⅔, 1}` and `y ∈ {0, ⅓, ⅔, 1}`. Their 16 intersections are the calibration
+points — **12 on the perimeter** (4 corners + 2 per side) plus **4 interior crossings** at
+the centre of the "#". Click all 16 in the feed.
+
+The 4 interior points matter: a polynomial constrained only on the boundary can wiggle in
+the interior, so the centre crossings pin the fit where edge points can't.
+
+**Capture by jog-and-mark, not ruler-and-guess:** jog the spindle to each target
+coordinate and mark at the tip, so the commanded machine coordinate is exact and the mark
+sits exactly under the tip. Then click each mark in the video.
+
+For the 2440×1220 bed the 16 targets are every combination of:
+
+```
+X: 0    813.3   1626.7   2440
+Y: 0    406.7    813.3    1220
+```
+
+Interior four: (813.3, 406.7), (1626.7, 406.7), (813.3, 813.3), (1626.7, 813.3).
+
+This 16-point spread also clears the degree-3 minimum, so escalating from degree 2 to 3
+needs no extra capture.
+
 ## Migration
 
 The committed default calibration in `dist/config.json` is a homography and **will not