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.
grokbot_ty
b3a9fdf330