diff --git a/frontend/src/capture/photosReadyLineup.test.ts b/frontend/src/capture/photosReadyLineup.test.ts
new file mode 100644
index 0000000..42d9c5c
--- /dev/null
+++ b/frontend/src/capture/photosReadyLineup.test.ts
@@ -0,0 +1,207 @@
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { test } from 'node:test';
+import type { BatchPhotoResult } from './capture.types';
+import {
+  buildPhotosReadyLineup,
+  type PhotosReadyLineupItem,
+} from './photosReadyLineup';
+
+const here = dirname(fileURLToPath(import.meta.url));
+
+function png(label: string): Blob {
+  return new Blob([label], { type: 'image/png' });
+}
+
+function jpeg(label: string): Blob {
+  return new Blob([label], { type: 'image/jpeg' });
+}
+
+function roles(items: PhotosReadyLineupItem[]): string[] {
+  return items.map((it) =>
+    it.kind === 'png' ? `png:${it.role}` : `error:${it.batchIndex}`,
+  );
+}
+
+// --- pure helper: lineup order + error slots (#64) ---
+
+test('test(photos): lineup is clean cal → overlay → other batch PNGs in capture order (#64)', () => {
+  const clean = png('clean-cal');
+  const overlay = png('overlay-cal');
+  const sib0 = png('sib-0');
+  const sib2 = png('sib-2');
+  const batchApply: BatchPhotoResult[] = [
+    { status: 'ok', inventoryPng: sib0 },
+    null, // calibration at index 1
+    { status: 'ok', inventoryPng: sib2 },
+  ];
+
+  const lineup = buildPhotosReadyLineup({
+    inventoryPng: clean,
+    overlayPng: overlay,
+    batchApply,
+  });
+
+  assert.deepEqual(roles(lineup), [
+    'png:calibration',
+    'png:overlay',
+    'png:batch',
+    'png:batch',
+  ]);
+  assert.equal(lineup[0].kind, 'png');
+  if (lineup[0].kind === 'png') {
+    assert.equal(lineup[0].blob, clean);
+    assert.equal(lineup[0].role, 'calibration');
+  }
+  assert.equal(lineup[1].kind, 'png');
+  if (lineup[1].kind === 'png') {
+    assert.equal(lineup[1].blob, overlay);
+    assert.equal(lineup[1].role, 'overlay');
+  }
+  assert.equal(lineup[2].kind, 'png');
+  if (lineup[2].kind === 'png') {
+    assert.equal(lineup[2].blob, sib0);
+    assert.equal(lineup[2].batchIndex, 0);
+  }
+  assert.equal(lineup[3].kind, 'png');
+  if (lineup[3].kind === 'png') {
+    assert.equal(lineup[3].blob, sib2);
+    assert.equal(lineup[3].batchIndex, 2);
+  }
+});
+
+test('test(photos): single-photo batch lineup is clean + overlay only (#64)', () => {
+  const clean = png('only-clean');
+  const overlay = png('only-overlay');
+  const lineup = buildPhotosReadyLineup({
+    inventoryPng: clean,
+    overlayPng: overlay,
+    batchApply: [null],
+  });
+  assert.deepEqual(roles(lineup), ['png:calibration', 'png:overlay']);
+  assert.ok(lineup.every((it) => it.kind === 'png'));
+});
+
+test('test(photos): batchApply errors become clear error slots (empty mask / floor) (#64)', () => {
+  const clean = png('clean');
+  const overlay = png('overlay');
+  const ok = png('ok-sib');
+  const batchApply: BatchPhotoResult[] = [
+    null,
+    { status: 'error', message: 'empty mask: no slab pixels detected' },
+    { status: 'ok', inventoryPng: ok },
+    { status: 'error', message: 'PNG size floor: shorter side is below 800 px' },
+  ];
+
+  const lineup = buildPhotosReadyLineup({
+    inventoryPng: clean,
+    overlayPng: overlay,
+    batchApply,
+  });
+
+  assert.deepEqual(roles(lineup), [
+    'png:calibration',
+    'png:overlay',
+    'error:1',
+    'png:batch',
+    'error:3',
+  ]);
+
+  const err1 = lineup[2];
+  assert.equal(err1.kind, 'error');
+  if (err1.kind === 'error') {
+    assert.match(err1.message, /empty mask/i);
+    assert.equal(err1.batchIndex, 1);
+  }
+  const err3 = lineup[4];
+  assert.equal(err3.kind, 'error');
+  if (err3.kind === 'error') {
+    assert.match(err3.message, /floor|800/i);
+  }
+
+  // Successful sibling still appears after cal+overlay, in capture order among siblings
+  const okItem = lineup[3];
+  assert.equal(okItem.kind, 'png');
+  if (okItem.kind === 'png') {
+    assert.equal(okItem.blob, ok);
+    assert.equal(okItem.batchIndex, 2);
+  }
+});
+
+test('test(photos): lineup never uses source JPEG thumbs as inventory PNGs (#64)', () => {
+  const clean = png('clean');
+  const overlay = png('overlay');
+  const thumb = jpeg('source-thumb');
+  // Dishonest proxy would pass source thumbs; helper must only emit image/png blobs for png slots
+  const lineup = buildPhotosReadyLineup({
+    inventoryPng: clean,
+    overlayPng: overlay,
+    batchApply: [
+      null,
+      { status: 'ok', inventoryPng: png('sib') },
+    ],
+  });
+  for (const it of lineup) {
+    if (it.kind === 'png') {
+      assert.equal(it.blob.type, 'image/png', 'lineup PNG slots must be inventory PNGs');
+      assert.notEqual(it.blob, thumb);
+    }
+  }
+});
+
+test('test(photos): skip clean cal when sourceBelowFloor; keep overlay + siblings (#64)', () => {
+  const lineup = buildPhotosReadyLineup({
+    inventoryPng: png('below'),
+    overlayPng: png('overlay'),
+    batchApply: [null, { status: 'ok', inventoryPng: png('sib') }],
+    sourceBelowFloor: true,
+  });
+  assert.deepEqual(roles(lineup), ['png:overlay', 'png:batch']);
+});
+
+test('test(photos): empty/missing results yield empty lineup (#64)', () => {
+  assert.deepEqual(buildPhotosReadyLineup({}), []);
+  assert.deepEqual(
+    buildPhotosReadyLineup({ inventoryPng: null, overlayPng: null, batchApply: undefined }),
+    [],
+  );
+});
+
+// --- light UI / session wiring (#64) ---
+
+test('test(photos): MeasurePanel shows Photos ready lineup + M1 numbers + batch errors (#64)', () => {
+  const panel = readFileSync(join(here, 'MeasurePanel.svelte'), 'utf8');
+  assert.match(panel, /Photos ready|photos.?ready/i, 'end screen title after Calculate');
+  assert.match(panel, /buildPhotosReadyLineup/, 'must use pure lineup helper');
+  assert.match(panel, /Widths/, 'M1 widths still visible');
+  assert.match(panel, /Sqft/, 'M1 sqft still visible');
+  assert.match(panel, /Bdft/, 'M1 bdft still visible');
+  assert.match(panel, /Scale/, 'M1 scale still visible');
+  // Dishonest proxies
+  assert.ok(
+    !/lineup[\s\S]{0,200}state\.source\.blob/.test(panel),
+    'must not lineup source thumbs',
+  );
+  assert.match(
+    panel,
+    /error|batchApply|message/i,
+    'per-photo batchApply errors must surface in the panel',
+  );
+  assert.match(panel, /Back/, 'Back must still work from Photos ready');
+});
+
+test('test(photos): App still routes measure results through Measure (or PhotosReady) with Back (#64)', () => {
+  const app = readFileSync(join(here, '..', 'App.svelte'), 'utf8');
+  // Stay on Measure with results, or dedicated panel — either is fine
+  assert.ok(
+    /MeasurePanel/.test(app) || /PhotosReady/.test(app),
+    'App must render Photos-ready end screen',
+  );
+  // Must not invent a Mask loop for the lineup
+  assert.ok(
+    !/\{\s*#each\s+[^}]*photos[^}]*\}[\s\S]*MaskPanel/i.test(app),
+    'App must not #each batch into Mask for Photos ready',
+  );
+});