feat: add walletkit-testkit crate for reusable e2e test helpers

[kilianglas]

Summary

Adds a new walletkit-testkit workspace crate that consolidates the end-to-end / test-support logic previously triplicated across walletkit-core's integration tests, the walletkit-cli binary, and external consumers (e.g. orb-tools). walletkit-cli is migrated to consume it.

The motivation: other services that interact with the World ID protocol via WalletKit (e.g. orb-tools) currently re-implement WalletKit's storage providers and on-chain proof logic by copy-pasting from the CLI. This crate exposes that surface as a reusable library so consumers can depend on it directly (by git rev) instead of duplicating it.

walletkit-testkit is not published (publish = false): it ships staging URLs and pre-registered test-RP / test-issuer private keys, and is intended to be consumed by git revision.

What's in the crate

• env — TestEnv config struct centralizing all staging fixtures (RP id/key, on-chain verifier address, World Chain RPC, faux-issuer URL + schema, local issuer key + schema). TestEnv::default() = staging; every field is overridable.
• storage — reusable StorageProvider impls: InMemoryStorageProvider (ephemeral, XChaCha20Poly1305 keystore) and FsStorageProvider (filesystem, no-op keystore), plus store constructors and cleanup helpers.
• authenticator — init_authenticator (fs store + cached Groth16 + init_with_defaults) and register_account (init_or_register → leaf_index).
• issuer — two interchangeable issuance strategies producing a shared IssuedTestCredential: issue_faux_credential (hosted faux issuer, HTTP, schema 128) and issue_local_credential (local EdDSA, schema 47, deterministic / no service dependency).
• proof — build_test_request (RP-signed, TestEnv-parameterized) and verify_proof_onchain + VerifyItemResult (against the WorldIDVerifier contract).
• flow — generate_and_verify_test_proof: high-level convenience wiring the full register → init → issue → prove → verify pipeline for either issuance strategy.

CLI migration

walletkit-cli now consumes the testkit instead of carrying its own copies:

• Deleted provider.rs (storage now from walletkit_testkit::storage).
• Removed the sol\! verifier binding, VerifyItemResult, verify_proof_onchain, build_test_request, issue_test_credential, and all staging constants — imported from the testkit.
• CLI behaviour is unchanged; --rpc-url / --verifier-address are mapped onto a TestEnv.

Testing

• New #[ignore]d staging e2e tests (tests/e2e.rs) exercise both issuance paths end-to-end and confirm on-chain verification. Both pass against staging.
• All 17 existing walletkit-cli tests pass (including the generate-test-request tests that drive the migrated builder).
• cargo clippy --workspace --all-targets, cargo fmt --check, cargo doc, and cargo build --workspace are all green.

Follow-ups (out of scope)

• Migrate walletkit-core/tests to consume the testkit.
• Migrate orb-tools (PR Bump to world-id-protocol versions which disable zkey caching #218) to depend on walletkit-testkit by git rev and delete its copied modules.