docs: add capture user flow and rectangle length axis

docs/CAPTURE.md

@@ -2,25 +2,23 @@
 
 Runs in the browser on phone and PC.
 
-## Output
+## Flow
 
-From one approved reference photo:
+1. User adds one slab photo. First photo is top-down, lens parallel to the face.
+2. App draws a mask overlay on the source photo. Controls: sheet Auto / Green / Black, one sensitivity slider. Overlay updates as the slider moves.
+3. User adjusts until the mask edge sits on the slab, then approves the mask.
+4. App draws the minimum-area bounding rectangle of the mask and the length axis on the long side of that rectangle. User rotates the axis if it does not follow the slab length, then approves the axis.
+5. User enters length `L` in inches (1/8 in steps) and thickness (1/8 in steps).
+6. App sets `scale = longest_axis_px / L`.
+7. App computes widths, sqft, and bdft, and writes the inventory PNG.
+8. User sees the PNG and the numbers.
 
-- Inventory PNG: transparent, 3:4, slab centered, 80% fill at extremes, shorter side at least 1600px when the source allows
+## Measurements
-- Widths (min, max, avg) sampled every 6 in along the length axis
+
+- Widths: min, max, avg, sampled every 6 in along the approved length axis, perpendicular to it
 - `sqft = (mask_pixels / scale²) / 144` (2 decimals)
 - `bdft = sqft × thickness_in` (2 decimals)
 
-`scale = longest_axis_px / L` where `L` is user length in inches (1/8 in steps).
+## Inventory PNG
 
-## Controls
+Transparent, 3:4, slab centered, 80% fill at extremes. Shorter side at least 1600px when the source allows. Source below that floor requires a new photo.
-
-- Sheet: Auto, Green, or Black
-- Sensitivity: one slider
-- Overlay on the source photo, updated as the slider moves
-
-User approves the edge, then the PNG and measurements run.
-
-Length axis comes from the mask. User may rotate it. Thickness is a user 1/8 in pick.
-
-Source below the PNG size floor requires a new photo.