[playeriv65]

Shift-clicking a boat-for-emerald trade currently sells only one boat, even when more boats are available in the inventory. The same problem affects prices above half a stack and unstackable items in the second payment slot.

The refill runs at the start of quickMoveStack. After a trade exhausts its payment, vanilla clears the result slot and ends the shift-click loop before quickMoveStack can run again.

This change wraps quickMoveStack, retains the active offer, and refills payment slots after a successful trade, before vanilla checks whether to continue. It leaves the actual transaction to vanilla and preserves offer stock limits. Failed output moves do not trigger a refill.

Validation on Fabric / Minecraft 26.3:

• Before the fix: six boats yield only one emerald; high-cost and unstackable second-payment cases also fail.
• After the fix: all six self-test cases pass: ordinary stackable payments, boats, offer stock exhaustion, 40-emerald payments, an unstackable second payment, and a full inventory.
• BULKTRADE_SELFTEST=true ./gradlew :fabric:runServer :fabric:build completes with [SELFTEST] RESULT PASS.

The self-tests run against the real merchant menu with the mixin applied, using the existing shift-click test helper. Other Minecraft versions and NeoForge have not been runtime-tested locally. No manual client playtest has been performed.