fix(photos): shared EXIF-oriented decode for axis/overlay space (#72)

PhotoStage used new Image() while mask worker and calculate used bare
createImageBitmap; Orientation-8 matched-set JPEGs can disagree across
those paths so the approved pink axis is not the overlay (ux,uy).

One decodeOrientedBitmap helper always requests imageOrientation
from-image (with bare fallback), wired into PhotoStage, mask.worker,
and session calculate/siblings.

frontend/src/png/overlay.test.ts

@@ -545,6 +545,43 @@ function tiltedAxis25Fixture() {
   return { W, H, bitset, bbox, ux0, uy0, rx0, ry0, halfL, cx, cy };
 }
 
+
+test('buildOverlayPlan: angleDeg=90 vertical long side → width left→right within 2° of horizontal (#72)', () => {
+  // Upright slab: approved length axis is vertical (angleDeg 90). Width marks must
+  // run left→right ≈ horizontal (atan2 ≈ 0 or ±π), not slanted with the photo.
+  const W = 200, H = 400;
+  const bitset = new Uint8Array(W * H);
+  for (let y = 40; y <= 360; y++) {
+    for (let x = 60; x <= 140; x++) bitset[y * W + x] = 1;
+  }
+  const prev = minAreaRect(bitset, W, H)!;
+  const bbox = orientedRect(prev, 90);
+  assert.equal(bbox.angleDeg, 90);
+  const scale = 10; // 320px span ≈ 32"
+  const plan = buildOverlayPlan({
+    bitset,
+    widthPx: W,
+    heightPx: H,
+    bbox,
+    scale,
+    lengthIn: 32,
+    lengthLabel: '32"',
+    mapToInventory: (x, y) => [x, y],
+  });
+  assert.ok(plan.widths.length > 0, 'expected width stations');
+  for (const w of plan.widths) {
+    const ang = Math.atan2(w.right[1] - w.left[1], w.right[0] - w.left[0]);
+    // Fold to [-90, 90] so left→right and right→left both count as horizontal.
+    let deg = (ang * 180) / Math.PI;
+    if (deg > 90) deg -= 180;
+    if (deg < -90) deg += 180;
+    assert.ok(
+      Math.abs(deg) <= 2,
+      `width station ${w.stationIn} left→right must be within 2° of horizontal (atan2 deg=${deg.toFixed(2)}; left=${w.left} right=${w.right})`,
+    );
+  }
+});
+
 test('buildOverlayPlan: length a→b || approved (ux,uy); width left→right || (rx,ry) (#70)', () => {
   const { W, H, bitset, bbox, cx, cy } = tiltedAxis25Fixture();
   const scale = 10;