refactor: add missing IWorldIDVerifierV2 interface

[kilianglas]

Summary

WorldIDVerifierV2 is the only versioned core contract without a matching versioned interface — WorldIDRegistryV2 pairs with IWorldIDRegistryV2 is IWorldIDRegistry, while the verifier V2 declares its InvalidAction error directly on the contract. This adds the missing IWorldIDVerifierV2 so the verifier follows the same pattern, and so future verifier versions (e.g. a V3 adding new entry points) have a proper interface chain to extend.

• New IWorldIDVerifierV2 is IWorldIDVerifier: declares InvalidAction and documents V2's action-prefix semantics (verify requires MSB 0x00, verifySession requires 0x02).
• WorldIDVerifierV2 is IWorldIDVerifierV2, WorldIDVerifier (mirrors WorldIDRegistryV2): local error declaration removed; overrides now name both bases (override(IWorldIDVerifier, WorldIDVerifier)), as required by the diamond inheritance and consistent with the registry.
• Tests reference the error via the interface (IWorldIDVerifierV2.InvalidAction.selector), matching the registry test idiom.
• Also fixes the stale @title WorldIDVerifier on the V2 contract.

No behavior change: same functions, same checks, same error selector.

Test plan

• cd contracts && forge test — 331 tests green

[kilianglas]

Superseded: the V2 interface will be introduced together with verifyWithSession in #862 instead.