feat(photos): pick calibration photo before Mask (#60)

Add pickCalibration state/UI over batch thumbs. Continue from batch opens
pick (including single-photo confirm). User pick enters M1 Mask on that
Source only; draft stores calibrationIndex. Removes first-photo proxy.

frontend/src/capture/session.ts

@@ -30,8 +30,10 @@ export interface CaptureSession {
   /** Alias: same as addToBatch (no longer jumps to mask). */
   addPhoto(file: File): Promise<void>;
   removeFromBatch(index: number): void;
-  /** Advance when batch has ≥1 photo. M1 bridge: mask on first photo. */
+  /** Advance when batch has ≥1 photo → pickCalibration. */
   continueBatch(): void;
+  /** User picks which batch photo is the calibration reference → Mask on that Source. */
+  pickCalibration(index: number): void;
   retakes(): void;
   setSensitivity(n: number): void;
   setSheet(s: Sheet): void;
@@ -63,6 +65,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
   const busy = writable(false);
   let state: Capture = { k: 'batch', photos: [] };
   let batchPhotos: Source[] = [];
+  let calibrationIndex: number | null = null;
   let worker: Worker | null = null;
   let jobId = 0;
   let timer: ReturnType<typeof setTimeout> | null = null;
@@ -173,9 +176,25 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
         inventoryPng: null,
       };
     }
+    if (state.k === 'pickCalibration') {
+      return {
+        version: 1,
+        stage: 'pickCalibration',
+        batch: state.photos,
+        source: state.photos[0],
+        maskParams: DEFAULT_MASK_PARAMS,
+        mask: { slabPx: 0, bbox: EMPTY_BBOX, slabBounds: EMPTY_ABOX },
+        lengthEighths: 0,
+        thicknessEighths: 0,
+        results: null,
+        inventoryPng: null,
+      };
+    }
+    const calIdx = calibrationIndex ?? undefined;
     if (state.k === 'axis') {
       return {
         version: 1, stage: 'axis', batch: batchPhotos.length ? batchPhotos : [state.source],
+        calibrationIndex: calIdx,
         source: state.source, maskParams: state.approvedMask.params,
         mask: strip(state.approvedMask), axisBbox: state.axis.bbox,
         lengthEighths: 0, thicknessEighths: 0, results: null, inventoryPng: null,
@@ -185,6 +204,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
       const { inventoryPng: _drop, ...rest } = state.results ?? ({} as Measures);
       return {
         version: 1, stage: 'measure', batch: batchPhotos.length ? batchPhotos : [state.source],
+        calibrationIndex: calIdx,
         source: state.source, maskParams: state.approvedMask.params,
         mask: strip(state.approvedMask), axisBbox: state.axis.bbox,
         lengthEighths: state.lengthEighths, thicknessEighths: state.thicknessEighths,
@@ -195,11 +215,22 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
     if (state.k === 'mask' && state.overlay) {
       return {
         version: 1, stage: 'mask', batch: batchPhotos.length ? batchPhotos : [state.source],
+        calibrationIndex: calIdx,
         source: state.source, maskParams: state.overlay.params,
         mask: strip(state.overlay),
         lengthEighths: 0, thicknessEighths: 0, results: null, inventoryPng: null,
       };
     }
+    // Mask before first overlay settles: still persist calibration choice + batch.
+    if (state.k === 'mask') {
+      return {
+        version: 1, stage: 'mask', batch: batchPhotos.length ? batchPhotos : [state.source],
+        calibrationIndex: calIdx,
+        source: state.source, maskParams: state.control,
+        mask: { slabPx: 0, bbox: EMPTY_BBOX, slabBounds: EMPTY_ABOX },
+        lengthEighths: 0, thicknessEighths: 0, results: null, inventoryPng: null,
+      };
+    }
     return null;
   };
 
@@ -252,11 +283,19 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
 
   const restoreFromDraft = async (d: Draft) => {
     batchPhotos = batchPhotosOf(d);
+    calibrationIndex = typeof d.calibrationIndex === 'number' ? d.calibrationIndex : null;
     if (d.stage === 'batch') {
       teardownWorker();
+      calibrationIndex = null;
       set({ k: 'batch', photos: [...batchPhotos] });
       return;
     }
+    if (d.stage === 'pickCalibration') {
+      teardownWorker();
+      calibrationIndex = null;
+      set({ k: 'pickCalibration', photos: [...batchPhotos] });
+      return;
+    }
     const source: Source = { widthPx: d.source.widthPx, heightPx: d.source.heightPx, blob: d.source.blob };
     if (d.stage === 'mask') {
       teardownWorker();
@@ -289,9 +328,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
     }
   };
 
-  const enterMaskFromBatch = () => {
-    if (batchPhotos.length === 0) return;
-    const source = batchPhotos[0];
+  const enterMaskOnSource = (source: Source) => {
     teardownWorker();
     set({
       k: 'mask',
@@ -324,6 +361,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
       if (state.k !== 'resume') return;
       teardownWorker();
       batchPhotos = [];
+      calibrationIndex = null;
       await drafts.clearDraft();
       set({ k: 'batch', photos: [] });
     },
@@ -361,11 +399,23 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
     continueBatch() {
       if (state.k !== 'batch' || state.photos.length === 0) return;
       batchPhotos = [...state.photos];
-      enterMaskFromBatch();
+      calibrationIndex = null;
+      teardownWorker();
+      set({ k: 'pickCalibration', photos: [...batchPhotos] });
+      void persist();
+    },
+
+    pickCalibration(index: number) {
+      if (state.k !== 'pickCalibration') return;
+      if (index < 0 || index >= state.photos.length) return;
+      batchPhotos = [...state.photos];
+      calibrationIndex = index;
+      enterMaskOnSource(batchPhotos[index]);
     },
 
     retakes() {
       teardownWorker();
+      calibrationIndex = null;
       // Return to the batch (photos kept) so the user can add/replace before continuing.
       set({ k: 'batch', photos: [...batchPhotos] });
       void persist();
@@ -448,10 +498,15 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
     },
 
     back() {
-      if (state.k === 'mask') {
-        teardownWorker();
+      if (state.k === 'pickCalibration') {
+        calibrationIndex = null;
         set({ k: 'batch', photos: [...batchPhotos] });
         void persist();
+      } else if (state.k === 'mask') {
+        teardownWorker();
+        calibrationIndex = null;
+        set({ k: 'pickCalibration', photos: [...batchPhotos] });
+        void persist();
       } else if (state.k === 'axis') {
         // back to mask keeps the last approved mask as the live overlay
         const m = state.approvedMask;