fix(review): #90 photo strip — blob: URL for Primary photo, checkerboard thumbs

Bug #1: the first/Primary (calibration) photo rendered broken because its
in-memory capture Blob was assigned to <img src> directly (String(Blob)=
"object Blob"). resolvePhotoUrl() now mints a blob: object URL (revoked on
unload) and falls back to the stored photo endpoint when there is no blob.
Bug #2: strip thumbnails used a flat dark bg; now carry the gray checkerboard
from the Capture lineup (MeasurePanel) so transparent inventory PNGs read the
same on both screens.
Also harden missingMinimums to filter kept internally (pre-existing test 7
failed on the clean base because it passed un-kept items).

frontend/src/create/review/lineup.ts

@@ -42,7 +42,8 @@ export function moveItem<T>(arr: T[], from: number, to: number): T[] {
  * clean calibration photo and one dimension-overlay copy. Returns the missing
  * role(s) so the UI can name them in a nudge.
  */
-export function missingMinimums(kept: LineupItem[]): PhotoRole[] {
+export function missingMinimums(items: LineupItem[]): PhotoRole[] {
+  const kept = items.filter((it) => it.kept);
   const missing: PhotoRole[] = [];
   if (!kept.some((it) => it.role === 'calibration')) missing.push('calibration');
   if (!kept.some((it) => it.role === 'overlay')) missing.push('overlay');
@@ -60,3 +61,22 @@ export function roleLabel(role: PhotoRole): string {
   if (role === 'overlay') return 'its dimension-overlay copy';
   return 'a batch photo';
 }
+
+/**
+ * Resolve the <img> src for a strip photo. The in-memory capture blob (fresh
+ * photo) must become a `blob:` object URL — assigning a raw Blob (or
+ * `String(blob)` = "object Blob") to `src` renders a broken image. That was
+ * bug #1: the first/Primary (calibration) photo is the one carrying a blob.
+ * When there is no usable blob, fall back to the stored photo endpoint.
+ *
+ * Pure + injectable: `createObjectUrl` defaults to `URL.createObjectURL` so it
+ * is unit-testable without a real DOM/Blob.
+ */
+export function resolvePhotoUrl(
+  blob: Blob | null,
+  endpoint: string,
+  createObjectUrl: (b: Blob) => string = (b) => URL.createObjectURL(b),
+): string {
+  if (blob && blob.size > 0) return createObjectUrl(blob);
+  return endpoint;
+}