Prod token test: verify beacon implementations have code

[thedavidmeister]

The beacon implementations (StoxReceipt, StoxReceiptVault, StoxWrappedTokenVault) must have code on-chain. Add a fork test asserting implementation().code.length > 0 for each beacon used by prod tokens.

This would catch an implementation being selfdestructed (post-Cancun this is less likely but still worth verifying).

[thedavidmeister]

Subsumed by what landed in #109 / #116.

LibProdTokensBaseTest.checkTokenSet now asserts LibExtrospectERC1967BeaconProxy.isBeaconImplementationBytecode(beacon, PROD_*_IMPLEMENTATION_BASE_CODEHASH_V1) for each of the three beacons. That comparison pins the runtime keccak against an in-repo non-empty codehash constant — if any implementation lost its code, keccak256(impl.code) would be keccak256("") and the existing assertion would fail with the right message.

Post-Cancun SELFDESTRUCT no longer removes deployed code (only clears balance), so the original threat model the issue cited has also narrowed.