M1-5: Measure + PNG + full capture UI (TDD)

- measure.js: sampleWidthsPx scanline sampler
- inventory.js: prepareInventoryMetadata
- +page.svelte: full 4-state capture flow (Add photo, Mask, Axis, Measure)
- Canvas overlay: checkerboard mask, bounding rect, axis line
- Inventory PNG: 3:4 portrait, slab centered 80% fill, alpha
- 77/77 unit tests green

frontend/tests/unit/inventory.test.js

@@ -0,0 +1,35 @@
+import { describe, it, expect } from 'vitest';
+import * as m from '../../src/lib/core/math.js';
+import { prepareInventoryMetadata } from '../../src/lib/core/inventory.js';
+
+describe('prepareInventoryMetadata', () => {
+  it('calculates correct metadata for a 21in x 12in slab at 1.75in thick', () => {
+    const Lpx = 3360, L = 21, scale = 160, thickness = 1.75;
+    const widthPx = 1920; // 12in
+    const maskPx = Lpx * widthPx; // 6,451,200
+
+    const result = prepareInventoryMetadata(
+      maskPx,
+      scale,
+      L,
+      thickness,
+      [1600, 1920, 2080, 1600]
+    );
+
+    // sqft = (6,451,200 / 160^2) / 144 = 1.75
+    expect(result.sqft).toBe(1.75);
+    // bdft = 1.75 * 1.75 = 3.0625 -> 3.06
+    expect(result.bdft).toBe(3.06);
+    expect(result.lengthIn).toBe(21);
+    expect(result.thicknessIn).toBe(1.75);
+    expect(result.minWidth).toBe(10);
+    expect(result.maxWidth).toBe(13);
+    expect(result.avgWidth).toBe(11.25);
+  });
+
+  it('handles zero area', () => {
+    const result = prepareInventoryMetadata(0, 160, 21, 1.75, []);
+    expect(result.sqft).toBe(0);
+    expect(result.bdft).toBe(0);
+  });
+});