docs: add live CAPTURE spec on doc-simplification

docs/CAPTURE.md

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+# Capture
+
+Client-side. Phone and PC. This is the hard part of Slice 1.
+
+## Goal
+
+Isolate just the slab, with quick visual feedback, then produce:
+
+1. A uniform inventory PNG for the product page
+2. Filterable sizes and board feet from that **reference** photo
+
+The user’s eye on the overlay is the quality gate. There is no confidence score on the mask.
+
+## Controls (Slice 1)
+
+- Sheet: Auto / Green / Black, only if the background is not obvious
+- **One** sensitivity slider
+- Overlay on the **source** photo, live as the slider moves
+- No Apply button, no server round-trip to tune the cut
+
+If the edge is still wrong after the slider, retake. Extra sliders (edge offset, feather, and so on) are a fallback after real slabs prove one control is not enough. Do not ship four sliders up front.
+
+## After the user approves the edge
+
+Then — and only then — run the rest on the client:
+
+- Length axis from the mask; user can rotate / confirm if it is skewed
+- User length `L` (inches, 1/8") is the scale: `scale = longest_axis_px / L`
+- Width samples every 6" along that axis (min / max / avg for the listing, not for bdft)
+- `sqft = (mask_pixels / scale²) / 144` (2 decimals)
+- `bdft = sqft × thickness_in` (2 decimals). Identity: 1 sqft at 1" = 1 bdft
+- Inventory PNG: transparent, 3:4 portrait, slab centered, 80% fill at the extremes, shorter side ≥ 1600px when the source allows
+- Undersized source: warn and require retake
+
+User-entered length, thickness, SKU, and price always win over computed values.
+
+## Proof for this file
+
+On a real phone, after Approve, the processed PNG and the numbers appear. If Approve does nothing, capture is not done.
+
+Algorithm detail and test vectors: `docs/archive/TECH-SPEC-PIPELINE.md` (history). Live behavior is this file.