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/measure/measure.tape.test.ts

@@ -0,0 +1,18 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { minAreaRect, orientedRect } from '../geometry/rect.ts';
+import { measure } from './measure.ts';
+
+test('#99: tape span drives scale, not maskSpanAlong', () => {
+  const W = 480, H = 100;
+  const bitset = new Uint8Array(W * H).fill(1);
+  const box = orientedRect(minAreaRect(bitset, W, H)!, 0);
+  // Declared L = 48"; tape span = 240 px → 5 px/in (not the full 480 mask span).
+  const m = measure(bitset, W, H, box, 384, 8, W * H, {
+    start: [100, 50],
+    ux: 1,
+    uy: 0,
+    spanPx: 240,
+  });
+  assert.ok(Math.abs(m.scale - 5) < 1e-9, `scale ${m.scale}`);
+});

frontend/src/measure/measure.ts

@@ -6,8 +6,19 @@ export function axisEndpoints(bbox: BBox): { start: [number, number]; end: [numb
   return { start: bbox.corners[3], end: bbox.corners[0] };
 }
 
+export interface TapeAxis {
+  /** Image-space tape start (tick A). */
+  start: [number, number];
+  /** Image-space unit direction tick A → tick B. */
+  ux: number;
+  uy: number;
+  /** Image-space distance between ticks (owner-declared tape length in px). */
+  spanPx: number;
+}
+
 // Wood end→end along the approved direction: project the mask onto the axis
 // unit vector and return the segment on the bbox center line covering that span.
+// Used only as a fallback / sanity path when no tape ticks are present (#99).
 function maskAxisSegment(
   bitset: Uint8Array, w: number, h: number, bbox: BBox,
 ): { start: [number, number]; end: [number, number]; span: number; ux: number; uy: number } {
@@ -71,16 +82,38 @@ function sampleWidths(
   return out;
 }
 
+/**
+ * @param tape When provided (#99), scale = tape.spanPx / L and width stations
+ *   walk from the tick-A endpoint. maskSpanAlong is not used for scale.
+ */
 export function measure(
   bitset: Uint8Array, w: number, h: number,
   bbox: BBox,
   lengthEighths: Eighths, thicknessEighths: Eighths,
   slabPx: number,
+  tape?: TapeAxis | null,
 ): Omit<Measures, 'inventoryPng'> & { sourceBelowFloor: false } {
   const lengthIn = lengthEighths / 8;
-  const seg = maskAxisSegment(bitset, w, h, bbox);
-  const scale = (seg.span || maskSpanAlong(bitset, w, h, seg.ux, seg.uy)) / lengthIn;
-  const every6In = sampleWidths(bitset, w, h, seg.start, seg.ux, seg.uy, scale, lengthIn);
+  let start: [number, number];
+  let ux: number;
+  let uy: number;
+  let scale: number;
+  if (tape && tape.spanPx > 0 && lengthIn > 0) {
+    start = tape.start;
+    ux = tape.ux;
+    uy = tape.uy;
+    scale = tape.spanPx / lengthIn;
+    // Optional sanity: maskSpanAlong remains available for diagnostics only.
+    void maskSpanAlong;
+  } else {
+    const seg = maskAxisSegment(bitset, w, h, bbox);
+    // Legacy fallback when no tape ticks — keep prior behaviour for old drafts.
+    scale = (seg.span || maskSpanAlong(bitset, w, h, seg.ux, seg.uy)) / lengthIn;
+    start = seg.start;
+    ux = seg.ux;
+    uy = seg.uy;
+  }
+  const every6In = sampleWidths(bitset, w, h, start, ux, uy, scale, lengthIn);
   let widths: Widths;
   if (every6In.length > 0) {
     const min = Math.min(...every6In);