feat(photos): keep M1 mask/axis/measure on calibration pick (#61)

Document single-source M1 path after pickCalibration: Mask/Axis/Measure/
Calculate use the chosen Source only; batch siblings stay draft-untouched
until #62 apply. Tag encode-fallback inventory Blob as image/png (CAPTURE.md).

frontend/src/capture/session.ts

@@ -405,6 +405,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
       void persist();
     },
 
+    // #61: enter M1 Mask on the chosen calibration Source only — never loop batch.
     pickCalibration(index: number) {
       if (state.k !== 'pickCalibration') return;
       if (index < 0 || index >= state.photos.length) return;
@@ -434,6 +435,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
       runPass();
     },
 
+    // #61: Axis uses the same calibration source; draft.batch siblings stay untouched.
     approveMask() {
       if (state.k !== 'mask' || !state.overlay || state.overlay.slabPx === 0) return;
       teardownWorker();
@@ -471,6 +473,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
       void persist();
     },
 
+    // #61: M1 Calculate on the calibration source only (batch apply is #62).
     async calculate() {
       if (state.k !== 'measure') return;
       const { source, approvedMask, axis, lengthEighths, thicknessEighths } = state;
@@ -487,7 +490,8 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
         pngBlob = r.blob;
         belowFloor = r.sourceBelowFloor;
       } catch {
-        pngBlob = new Blob();
+        // CAPTURE.md: inventory is image/png even when encode is unavailable.
+        pngBlob = new Blob([], { type: 'image/png' });
       }
       bitmap.close();
       if (pngUrl) URL.revokeObjectURL(pngUrl);