test(capture): require mask alpha in inventory PNG compose

Why
Calculate showed accurate numbers but the inventory PNG kept the sheet
background. Measure already uses approvedMask.bitset; encode did not.

Scope
Add composeInventoryFrame and inventory.test.ts alpha/crop proofs.
Wire encodeInventoryPng({ bitmap, mask, source }) at calculate().
This commit still paints the slabBounds AABB without the bitset gate so
the alpha contract fails until the follow-up fix.

Verification
node --import tsx --test src/png/inventory.test.ts
alpha contract fails (sheet green leaks); 3:4 / 80% / floor pass.

frontend/src/capture/session.ts

@@ -315,7 +315,7 @@ export function createCaptureSession(): CaptureSession {
       let pngBlob: Blob;
       let belowFloor = false;
       try {
-        const r = await encodeInventoryPng(bitmap, approvedMask.slabBounds);
+        const r = await encodeInventoryPng({ bitmap, mask: approvedMask, source });
         pngBlob = r.blob;
         belowFloor = r.sourceBelowFloor;
       } catch {