fix(capture): drop first and last 6in from width samples

sampleWidths used floor(L/6)+1 stations from t=0, so tapered tips
dominated min/max. Sample only interior stations at 6, 12, … ≤ L−6.
When L≤12 there is no interior span: every6In stays empty and
min/max/avg stay 0. CAPTURE.md Widths bullet matches.

sqft, bdft, scale, and inventory PNG are unchanged.

docs/CAPTURE.md

@@ -94,7 +94,7 @@ On the same photo, keep the approved mask in view and draw the minimum-area boun
 User enters length `L` in inches (1/8 in steps) and thickness (1/8 in steps), then presses Calculate.
 
 - `scale = longest_axis_px / L` where `longest_axis_px` is the mask/wood span along the approved length direction (slab pixels end-to-end on that axis), not the rectangle edge length. The rectangle sets direction only; user-entered `L` is wood end→end along that direction.
-- Widths: min, max, avg, sampled every 6 in along the approved length axis, perpendicular to it
+- Widths: min, max, avg from samples taken every 6 in along the approved length axis, perpendicular to it, after dropping the first and last 6 in of the span (bulk widths, not tips). If `L ≤ 12 in` there are no interior stations and all three are 0.
 - `sqft = (mask_pixels / scale²) / 144` (2 decimals)
 - `bdft = sqft × thickness_in` (2 decimals)
 - Crop: 3:4 portrait, slab centered, 80% fill at the widest and tallest points, transparent background