chore(testing): add Anvil-based EVM integration harness

[Markodiba]

Summary

Implements issue #219 (doc Issue #7 in docs/ethereum-expansion-issues.md): a local Anvil
(Foundry) devnet harness for EVM integration tests, giving EVM work the same local-devnet story
Stellar already has via StellarNetwork::Standalone
(crates/wallet-core/src/signer.rs).

• New octo-evm-test-support crate (crates/evm-test-support):
  • AnvilInstance: spawns anvil --port 0 per test (random free port, own process), parses the
    real bound port from its startup banner, confirms readiness by polling eth_chainId, and
    kills the process in Drop — including on panic, since Drop runs during unwind.
  • MockErc20: deploys a constructor-configurable-decimals ERC-20
    (contracts/MockERC20.sol, precompiled bytecode checked in as contracts/MockERC20.bin so
    only anvil, not solc, needs to be on PATH), with mint/transfer/balance_of helpers
    that produce real Transfer logs.
  • Reorg controls: snapshot() / revert_to(id) (anvil_snapshot/anvil_revert), mine(n)
    (evm_mine), set_base_fee(wei) (anvil_setNextBlockBaseFeePerGas) — the primitive feat(ingest): Confirmation depth and reorg handling for EVM deposits #222
    (reorg handling) depends on, since a reorg can't be induced on demand on a public testnet.
  • gate() mirrors the OCTO_LIVE_TESTS pattern in
    crates/api/tests/horizon_live_tests.rs: tests skip
    cleanly (not fail) unless OCTO_EVM_TESTS=1 and anvil is on PATH, so just test stays
    green with no Foundry installed.
• just test-evm recipe; CI installs Foundry (foundry-rs/foundry-toolchain@v1) and runs it.
• CONTRIBUTING.md: documents the optional Foundry install.

Scope note: the issue says "Depends on #218" (octo-evm-rpc, the resilient typed JSON-RPC
client) and suggests crates/evm-rpc/tests/common/anvil.rs, but neither crates/evm-core (#217)
nor crates/evm-rpc (#218) exist in this repo yet. The issue itself anticipates this and offers a
fallback — "or a small test-support crate if #221/#224 also need it" — which is what this PR does:
a standalone crate with its own minimal JSON-RPC client (src/rpc.rs, just enough to drive Anvil,
including the HTTP-200-with-error-member check #218 will also need) and no dependency on
unbuilt crates. Transactions go through plain eth_sendTransaction against Anvil's pre-unlocked
default accounts (derived from Anvil's own default deterministic mnemonic — no --mnemonic flag
needed), so no client-side secp256k1 signing dependency was pulled in; that's #217/#218's job, not
this harness's.

Test plan

• cargo build --workspace and cargo test --workspace — new crate's tests skip cleanly with
  no OCTO_EVM_TESTS/no Foundry, rest of the workspace unaffected.
• just test-evm (Foundry installed locally) — all pass:
  • anvil_boots_and_mining_advances_the_chain — boots Anvil, mine(3), asserts
    eth_blockNumber advances by exactly 3.
  • snapshot_and_revert_undoes_a_mined_transfer — deploys MockErc20, mines a Transfer,
    snapshots, mines more, reverts, asserts the transfer's receipt is gone — the reorg proof feat(ingest): Confirmation depth and reorg handling for EVM deposits #222
    needs.
  • mock_erc20_supports_usdc_dai_and_zero_decimal_configs — exercises 6/18/0 decimals end to end.
  • drop_kills_the_anvil_process_even_when_the_test_panics (#[cfg(unix)]) — panics inside
    catch_unwind with the instance moved in, then confirms via kill -0 that the process is
    dead.
  • suite_skips_cleanly_when_the_env_var_is_unset plus deterministic unit tests of the skip
    predicate in src/lib.rs (no PATH/env mutation, so no flakiness under parallel tests).
• Verified no orphan anvil processes after a full test run (pgrep anvil).
• cargo fmt --all -- --check, cargo clippy -p octo-evm-test-support --all-targets -- -D warnings.
• Selector-encoding unit tests cross-checked against cast keccak (Foundry) for the standard
  ERC-20 signatures.

closes #219