Fix showoff item cleanup on chunk unload and double-chest handling

[MLG-SERBUR]

Motivation

This PR started from a review request and then narrowed to only changes that are actually useful.

User prompt:
“review #34

tell me what it changes, and if each change is actually useful/needed or not.

then implement the definitely useful ones.”

Assistant response (summary):
Reviewed PR #34, split changes into useful vs unnecessary/risky, and implemented only the clearly useful fixes.

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Conversation Summary (Prompt → Response)

User prompt:
“if the shop has to be a chest or doublechest, and the stock has to fit in that chest or doublechest, how is it possible for it to exceed the 54 slots?”

Assistant response (summary):
Agreed this cap is not needed in practice for chest/double-chest constrained stock, and reverted that clamp change.

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User prompt:
“as for location#getBlock in getContainer - check the git history/blame to see if that's intentional. I have a feeling it might've been to ensure the blockstate is loaded and it's not pulling from cache?”

Assistant response (summary):
Checked history/blame and restored location.getBlock(); instead of removing it casually. Also investigated upstream API/server sources; evidence suggests it may be a legacy/defensive call rather than strictly required, but keeping it is safer without a focused refactor.

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User prompt:
“for the despawn items - what if the placed block is not a shop? ... wouldn't this miss shops that are turned into doublechests with a non-shop chest block?”

Assistant response (summary):
Investigated shop creation flow and double-chest resolution. Current logic using shopAPI.isShop(container) should still detect merged double chests when one side is already a shop, while avoiding non-shop cleanup.

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User prompt:
“print the commit where location#getblock was introduced. Also check if my prior question was correct or not...”

Assistant response (summary):
Identified introduction commit as f75f7058d8591b6d7b55179900867e97325054e0 and confirmed rationale analysis above.

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Final Scope in This PR

Kept (definitely useful)

• Fix ShowoffItem chunk-unload matching to compare world + chunk coordinates instead of chunk object identity.
• Fix double-chest despawn to remove both left and right showcase positions (instead of despawning left twice).
• Keep shop check gate so cleanup only runs for recognized shops.

Reverted / Not Included

• Removed 54-slot post-purchase inventory clamp (not needed under chest/double-chest stock constraints).
• Kept location.getBlock(); in ShopAPI#getContainer (restored after history/blame review).