feat(capture): shared crop_fill + tape geometry for #99

- One crop_fill (default 0.9) replaces duplicated FILL in inventory/overlay
- Thin localStorage settings store; crop % only moves long-axis end insets
- Pure tapeGeometry: fixed stage line, stage↔image map, tick span → px
- measure() prefers tape tick span for scale; maskSpanAlong demoted
- rotateCalibration bakes cal tilt into inventory + overlay only
- Fix stale 80%-fill comment on slabBounds

frontend/src/geometry/rect.ts

@@ -115,6 +115,8 @@ export function orientedRect(prev: BBox, angleDeg: number): BBox {
 
 // Project every slab pixel onto the unit vector (ux, uy); return max − min
 // extent in px. 0 when the mask is empty.
+// #99: demoted to an optional sanity check — declared L comes from tape tick
+// distance in image space, not this projected guess.
 export function maskSpanAlong(bitset: Uint8Array, w: number, h: number, ux: number, uy: number): number {
   let min = Infinity, max = -Infinity;
   for (let y = 0; y < h; y++) {