Test helper API

Reusable helpers for writing fast, reliable Anvil mainnet-fork tests. These live under eth_defi.testing rather than tests so they can be imported from test suites in this repository and in downstream projects.

The canonical, authoritative description of the shared-fork pattern — fixed fork block, shared Anvil process, warm RPC cache — is the module docstring of eth_defi.testing.anvil_fork_pool. eth_defi/testing/README.md in the source tree has the step-by-step how-to, and docs/README-test-suite-performance.md the background and measurements.

Three caching layers cooperate to keep fork tests off the upstream archive node:

Shared forks — eth_defi.testing.anvil_fork_pool reuses one Anvil process per (chain, block, launch config), probes a reused fork for liveness and recycles it if it has wedged.
Fork state cache — eth_defi.testing.rpc_cache seeds Anvil’s on-disk fork RPC cache (~/.foundry/cache/rpc) from cache files committed to the repository, so a cold runner starts warm. Anvil only flushes that cache on a graceful shutdown, which is why eth_defi.provider.anvil.AnvilLaunch.close() terminates it gently.
Token metadata cache — eth_defi.testing.token_cache installs a committed eth_defi.token.TokenDiskCache as the default vault token cache, so vaults do not re-resolve token addresses and metadata over RPC in every process.

Fixed fork blocks shared by the characterisation tests are defined in eth_defi.testing.fork_blocks, and per-test EVM state isolation is provided by eth_defi.testing.evm_snapshot_fixture.

`eth_defi.testing.anvil_fork_pool`	Session-scoped Anvil fork pool for reusing forks across test modules.
`eth_defi.testing.fork_blocks`	Canonical per-chain fork blocks for shared-fork characterisation tests.
`eth_defi.testing.rpc_cache`	Seed the Foundry (Anvil) fork RPC cache from repository-supplied defaults.
`eth_defi.testing.token_cache`	Ship a prebuilt ERC-20 / vault token cache so tests do not refetch it over RPC.
`eth_defi.testing.evm_snapshot_fixture`	EVM snapshot/revert helper for per-test isolation on module-scope Anvil forks.