diff --git a/frontend/src/capture/session.calibrate.test.ts b/frontend/src/capture/session.calibrate.test.ts
index 9542c5a..c6057d6 100644
--- a/frontend/src/capture/session.calibrate.test.ts
+++ b/frontend/src/capture/session.calibrate.test.ts
@@ -2,9 +2,9 @@ import assert from 'node:assert/strict';
 import { readFileSync } from 'node:fs';
 import { dirname, join } from 'node:path';
 import { fileURLToPath } from 'node:url';
-import { test, beforeEach } from 'node:test';
+import { test, beforeEach, afterEach } from 'node:test';
 import { get } from 'svelte/store';
-import type { Draft, Source } from './capture.types';
+import type { Draft, Mask, Source, WorkerIn, WorkerOut } from './capture.types';
 import { createCaptureSession, type DraftStore } from './session';
 
 const here = dirname(fileURLToPath(import.meta.url));
@@ -219,3 +219,317 @@ test('App wires pickCalibration state to PickCalibration panel', () => {
   assert.match(app, /PickCalibration/, 'App must import/render PickCalibration');
   assert.match(app, /pickCalibration/, 'App must branch on pickCalibration state');
 });
+
+// --- #61 M2-4: wire M1 mask/axis/measure onto the calibration pick ---
+
+const MASK_W = 40;
+const MASK_H = 20;
+
+/** Fake Worker so Node can drive Mask → Axis → Measure without canvas decode. */
+class FakeMaskWorker {
+  onmessage: ((ev: MessageEvent<WorkerOut>) => void) | null = null;
+  static inits: Blob[] = [];
+
+  constructor(_url: URL | string, _opts?: WorkerOptions) {}
+
+  postMessage(msg: WorkerIn) {
+    if (msg.kind === 'init') {
+      FakeMaskWorker.inits.push(msg.blob);
+      const id = msg.id;
+      queueMicrotask(() => {
+        this.onmessage?.({ data: { id, kind: 'decoded', w: MASK_W, h: MASK_H } } as MessageEvent<WorkerOut>);
+      });
+      return;
+    }
+    if (msg.kind === 'mask') {
+      const id = msg.id;
+      const bitset = new Uint8Array(MASK_W * MASK_H).fill(1);
+      const bbox = {
+        corners: [
+          [MASK_W, 0],
+          [MASK_W, MASK_H],
+          [0, MASK_H],
+          [0, 0],
+        ] as [number, number][],
+        longSidePx: MASK_W,
+        angleDeg: 0,
+      };
+      queueMicrotask(() => {
+        this.onmessage?.({
+          data: {
+            id,
+            kind: 'ready',
+            slabPx: MASK_W * MASK_H,
+            bitset,
+            bbox,
+            slabBounds: { x: 0, y: 0, w: MASK_W, h: MASK_H },
+          },
+        } as MessageEvent<WorkerOut>);
+      });
+      return;
+    }
+    // invalidate: no-op
+  }
+
+  terminate() {}
+}
+
+const RealWorker = globalThis.Worker;
+const RealCreateImageBitmap = globalThis.createImageBitmap;
+
+function installFakeMaskWorker() {
+  FakeMaskWorker.inits = [];
+  (globalThis as unknown as { Worker: typeof FakeMaskWorker }).Worker = FakeMaskWorker;
+}
+
+function restoreWorkerGlobals() {
+  if (RealWorker === undefined) {
+    delete (globalThis as unknown as { Worker?: unknown }).Worker;
+  } else {
+    globalThis.Worker = RealWorker;
+  }
+  if (RealCreateImageBitmap === undefined) {
+    delete (globalThis as unknown as { createImageBitmap?: unknown }).createImageBitmap;
+  } else {
+    globalThis.createImageBitmap = RealCreateImageBitmap;
+  }
+}
+
+async function settleOverlay(
+  s: ReturnType<typeof createCaptureSession>,
+  ms = 800,
+): Promise<Mask> {
+  const start = Date.now();
+  while (Date.now() - start < ms) {
+    const cap = get(s.capture);
+    if (cap.k === 'mask' && cap.overlay && cap.overlay.slabPx > 0) return cap.overlay;
+    await new Promise((r) => setTimeout(r, 5));
+  }
+  throw new Error('mask overlay never settled');
+}
+
+afterEach(() => {
+  restoreWorkerGlobals();
+});
+
+test('#61: after pick of index N≠0, Mask/Axis/Measure operate on that blob only', async () => {
+  installFakeMaskWorker();
+  const s = createCaptureSession({ draft: store });
+  const a = tinyJpeg('a');
+  const b = tinyJpeg('b');
+  const c = tinyJpeg('c');
+  await s.addToBatch(a);
+  await s.addToBatch(b);
+  await s.addToBatch(c);
+  s.continueBatch();
+  s.pickCalibration(1);
+
+  let cap = get(s.capture);
+  assert.equal(cap.k, 'mask');
+  if (cap.k === 'mask') {
+    assert.equal(cap.source.blob, b, 'Mask must open on picked photo index 1');
+    assert.notEqual(cap.source.blob, a);
+  }
+
+  // Kick an immediate mask pass (skip 2s sensitivity debounce).
+  s.setSheet('green');
+  await settleOverlay(s);
+
+  // Worker must have been inited only for the calibration blob — not a, not c.
+  assert.ok(FakeMaskWorker.inits.length >= 1, 'mask worker must init');
+  for (const blob of FakeMaskWorker.inits) {
+    assert.equal(blob, b, 'Mask worker must never init on non-calibration batch photos');
+  }
+
+  s.approveMask();
+  cap = get(s.capture);
+  assert.equal(cap.k, 'axis', 'approve Mask → Length axis');
+  if (cap.k === 'axis') {
+    assert.equal(cap.source.blob, b, 'Axis must keep the calibration source');
+  }
+
+  s.approveAxis();
+  cap = get(s.capture);
+  assert.equal(cap.k, 'measure', 'approve Axis → Measure');
+  if (cap.k === 'measure') {
+    assert.equal(cap.source.blob, b, 'Measure must keep the calibration source');
+  }
+
+  s.dispose();
+});
+
+test('#61: Calculate produces inventory PNG + M1 measures for calibration source only', async () => {
+  installFakeMaskWorker();
+  (globalThis as unknown as { createImageBitmap: typeof createImageBitmap }).createImageBitmap =
+    async () =>
+      ({
+        width: MASK_W,
+        height: MASK_H,
+        close() {},
+      }) as ImageBitmap;
+
+  const s = createCaptureSession({ draft: store });
+  const a = tinyJpeg('a');
+  const b = tinyJpeg('b');
+  await s.addToBatch(a);
+  await s.addToBatch(b);
+  s.continueBatch();
+  s.pickCalibration(1);
+  s.setSheet('black');
+  await settleOverlay(s);
+  s.approveMask();
+  s.approveAxis();
+
+  // L = 40 in (320 eighths), thickness = 2 in (16 eighths) — scale ≈ longest_axis_px / L
+  s.setLength(320);
+  s.setThickness(16);
+  await s.calculate();
+
+  const cap = get(s.capture);
+  assert.equal(cap.k, 'measure');
+  if (cap.k === 'measure') {
+    assert.equal(cap.source.blob, b);
+    assert.ok(cap.results, 'Calculate must produce results');
+    const r = cap.results!;
+    assert.ok(r.inventoryPng instanceof Blob, 'clean inventory PNG blob');
+    assert.equal(typeof r.scale, 'number');
+    assert.ok(r.scale > 0, 'scale px/in > 0');
+    assert.equal(typeof r.sqft, 'number');
+    assert.equal(typeof r.bdft, 'number');
+    // CAPTURE.md: bdft = sqft × thickness_in
+    const thicknessIn = 16 / 8;
+    assert.ok(
+      Math.abs(r.bdft - r.sqft * thicknessIn) < 0.011,
+      `bdft ${r.bdft} must equal sqft×thickness (${r.sqft}*${thicknessIn})`,
+    );
+    assert.ok(r.widths, 'widths present');
+    assert.equal(r.lengthEighths, 320);
+    assert.equal(r.thicknessEighths, 16);
+    assert.equal(cap.source.widthPx, MASK_W, 'measure source dims from calibration decode');
+  }
+
+  // Still only calibration blob through the mask worker.
+  for (const blob of FakeMaskWorker.inits) {
+    assert.equal(blob, b);
+  }
+
+  s.dispose();
+});
+
+test('#61: other batch photos stay in draft.batch untouched by mask approve', async () => {
+  installFakeMaskWorker();
+  const s = createCaptureSession({ draft: store });
+  const a = tinyJpeg('a');
+  const b = tinyJpeg('b');
+  const c = tinyJpeg('c');
+  await s.addToBatch(a);
+  await s.addToBatch(b);
+  await s.addToBatch(c);
+  s.continueBatch();
+  s.pickCalibration(1);
+  s.setSheet('auto');
+  await settleOverlay(s);
+
+  const before = store.peek();
+  assert.ok(before);
+  assert.equal((before!.batch ?? []).length, 3);
+  assert.equal(before!.batch![0].blob, a);
+  assert.equal(before!.batch![1].blob, b);
+  assert.equal(before!.batch![2].blob, c);
+
+  s.approveMask();
+  await new Promise((r) => setTimeout(r, 20));
+
+  const after = store.peek();
+  assert.ok(after);
+  assert.equal(after!.stage, 'axis');
+  assert.equal(after!.calibrationIndex, 1);
+  assert.equal(after!.source.blob, b, 'draft.source is calibration only');
+  assert.equal((after!.batch ?? []).length, 3, 'full batch retained');
+  assert.equal(after!.batch![0].blob, a, 'batch[0] untouched');
+  assert.equal(after!.batch![1].blob, b);
+  assert.equal(after!.batch![2].blob, c, 'batch[2] untouched — no Mask UI on other shots');
+
+  // Approve axis too — batch still untouched.
+  s.approveAxis();
+  await new Promise((r) => setTimeout(r, 20));
+  const meas = store.peek();
+  assert.equal(meas!.stage, 'measure');
+  assert.equal((meas!.batch ?? []).length, 3);
+  assert.equal(meas!.batch![0].blob, a);
+  assert.equal(meas!.batch![2].blob, c);
+  assert.equal(meas!.calibrationIndex, 1);
+
+  s.dispose();
+});
+
+test('#61: draft resume through measure restores calibration source (not photos[0])', async () => {
+  installFakeMaskWorker();
+  (globalThis as unknown as { createImageBitmap: typeof createImageBitmap }).createImageBitmap =
+    async () =>
+      ({
+        width: MASK_W,
+        height: MASK_H,
+        close() {},
+      }) as ImageBitmap;
+
+  const s1 = createCaptureSession({ draft: store });
+  const a = tinyJpeg('a');
+  const b = tinyJpeg('b');
+  await s1.addToBatch(a);
+  await s1.addToBatch(b);
+  s1.continueBatch();
+  s1.pickCalibration(1);
+  s1.setSheet('green');
+  await settleOverlay(s1);
+  s1.approveMask();
+  s1.approveAxis();
+  s1.setLength(320);
+  s1.setThickness(16);
+  await s1.calculate();
+  await new Promise((r) => setTimeout(r, 30));
+  s1.dispose();
+
+  FakeMaskWorker.inits = [];
+  const s2 = createCaptureSession({ draft: store });
+  await s2.resumeProbe();
+  assert.equal(get(s2.capture).k, 'resume');
+  s2.continueDraft();
+  // rederiveMask via FakeWorker
+  const start = Date.now();
+  let cap = get(s2.capture);
+  while (cap.k !== 'measure' && Date.now() - start < 1500) {
+    await new Promise((r) => setTimeout(r, 10));
+    cap = get(s2.capture);
+  }
+  assert.equal(cap.k, 'measure', 'Continue must restore Measure');
+  if (cap.k === 'measure') {
+    assert.equal(cap.source.blob, b, 'restored Measure source is calibration pick');
+    assert.ok(cap.results, 'results restored');
+  }
+  assert.equal(store.peek()?.calibrationIndex, 1);
+  // Resume rederive may re-init worker on calibration source only.
+  for (const blob of FakeMaskWorker.inits) {
+    assert.equal(blob, b, 'resume must not mask non-calibration batch photos');
+  }
+  s2.dispose();
+});
+
+test('#61: App runs single Mask/Axis/Measure panels (no per-batch Mask loop)', () => {
+  const app = readFileSync(join(here, '..', 'App.svelte'), 'utf8');
+  assert.match(app, /MaskPanel/);
+  assert.match(app, /AxisPanel/);
+  assert.match(app, /MeasurePanel/);
+  // Dishonest proxy: must not iterate batch photos into Mask UI.
+  assert.ok(
+    !/\{\s*#each\s+[^}]*photos[^}]*\}[\s\S]*MaskPanel/i.test(app),
+    'App must not #each batch photos into MaskPanel',
+  );
+  const sessionSrc = readFileSync(join(here, 'session.ts'), 'utf8');
+  assert.ok(
+    !/for\s*\([^)]*batch[^)]*\)[\s\S]{0,120}(enterMaskOnSource|approveMask|k:\s*'mask')/.test(
+      sessionSrc,
+    ),
+    'session must not loop Mask over every batch photo in this issue',
+  );
+});