refactor: share common code and test the precompiles end to end

[sphamjoli]

The scarcity and NFT-claims precompiles are already in place, but each had its own copy of the same guards and test scaffolding, revert reasons written out inline at each site, and the collection interface holding two contracts in one file. Nothing exercised them the way a real caller does, through a compiled contract running inside the virtual machine, so the behaviour that only shows up across that boundary was never tested.

This shares the common parts and adds that coverage. The guards and mock helpers move to a single support crate; both precompiles reuse, so they cannot drift apart. Small example contracts are now compiled at test time and run against the precompiles, covering a plain read, the delegate-call and value refusals, a contract that owns and gates its own collection, and a proxy that owns a collection under its own address, on both the EVM and PolkaVM backends. Revert reasons become named constants shared by the code and the tests, the collection interface is split so each file holds one interface, and the standard interface identifiers are checked against the compiler's own computation of them rather than trusted by hand.

Changes

• Extract the shared guards and mock helpers into a common support crate that both precompiles depend on, so the rules stay identical.
• Add a fixtures crate that compiles small Solidity examples to EVM and PolkaVM at test time, fetching the compilers on demand so no toolchain has to be installed by hand.
• Add end-to-end tests: a normal read, the delegate-call and value guards, a contract that owns and mints from its own collection, an OpenZeppelin-based proxy that owns a collection, and the same read verified on both backends.
• Cover native operations as tests: a native force transfer registers its destination so balanceOf and ownerOf agree, and native transfers and burns emit no ERC-721 log.
• Turn every revert reason into a named public constant shared by callers and tests.
• Split the collection interface so each file holds a single interface, and verify the ERC-165 and ERC-721 interface identifiers against a vendored OpenZeppelin fixture computed by the compiler.

[sphamjoli]

Hello @tindzk can you please have a look when you get time