test(storage): cover final slot byte

[MuncleUscles]

Summary

Adds two integration regressions at the final valid byte of a 2^32-byte storage slot:

• a raw guest storage_read at offset 2^32 - 1 must succeed;
• a deployed-contract write at the same offset must pass range validation and reach the independent locked-slot check (forbidden).

Both fixtures intentionally fail against the current ABI guard, which treats the valid end-exclusive range as an overflow. No production fix is included.

Why

The guard uses u32::checked_add, which rejects index = 2^32 - 1 with a one-byte buffer even though that accesses the slot's last valid byte.

Validation

• Fixture Python sources compile successfully.
• Full executor integration execution was not available in this isolated worktree; the expected outputs encode the intended current failure and fixed behavior.

Stacked on pr/v0.3/codex/wasmtime-maintenance.

[coderabbitai]

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