feat: add --megaeth EVM mode

[RealiCZ]

Summary

Adds --megaeth mode to forge test and forge coverage, routing execution through mega-evm 1.5.1 while preserving Foundry's test / coverage / trace UX.

Core integration

• Route MegaETH transactions through MegaEvm with a dedicated inspector fan-out for MegaCtx (tracer / log collector / revert diagnostic wired; cheatcodes and isolation intentionally skipped).
• Deploy MegaETH system contracts (Oracle, HP Timestamp, Keyless Deploy) idempotently: skip when code_hash already matches, preserve existing balance / nonce — mirrors mega-evme's pattern.
• Translate mega-evm ExecutionResult / HaltReason into their revm counterparts for downstream reporting.

Flag validation

New shared EvmOpts::validate_megaeth() rejects unsupported combinations at fail-fast CLI layer, before any network request:

Combination	Behavior
--megaeth --isolate	rejected — isolation not implemented
--megaeth --fork-url <URL>	rejected — no fork-aware external env
--megaeth --gas-report	rejected — gas report implies isolation
Inline forge-config: isolate = true + --megaeth	rejected at test time

Applied to both forge test and forge coverage. Coverage also now correctly forwards --isolate / --odyssey to the runner (previously silently dropped).

Test plan

• cargo nextest run -p forge --test cli -E 'test(~megaeth)'
• make test
• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features
• cargo nextest run -p forge --test cli megaeth_live_cross_validate --run-ignored=only — passes locally and installs mega-evme@1.5.1 from crates.io into target/mega-evme-1.5.1/ on first run

Note

Alloy version bump

• All alloy-* pinned to =1.0.23 — required by mega-evm 1.5.1's Cargo.toml.

[claude]

Review posted (see below)

[claude]

Placeholder - ignore

[claude]

test with backtick foo

[claude]

Review

Good overall structure. The validation layering (CLI → builder → inline-config) is thorough and the idempotent system contract deployment mirrors mega-evme correctly. A few issues worth addressing:

Must fix — Alloy exact-pin change needs justification (REVIEW.md rule)

All alloy deps were bumped from 1.0.22 to =1.0.23 (exact pin) and alloy-primitives from 1.2.1 to 1.3.1. REVIEW.md requires that changes to alloy explain the fork-specific need and test coverage. The PR description is silent on why. If mega-evm 1.5.1 requires these exact versions to avoid a diamond-dep conflict, that should be stated explicitly.

Must fix — MegaCtx is defined twice

pub type MegaCtx is declared in both crates/evm/core/src/evm.rs and crates/evm/evm/src/inspectors/stack.rs. The definitions are structurally identical today, but if they ever drift the compiler will not catch it — callers silently get the wrong type. The stack.rs definition should be removed and replaced with: use foundry_evm_core::evm::MegaCtx;

Should fix — Backend::is_megaeth is pub

All other Backend fields are private; this one exposes mutable access to any caller. A stray backend.is_megaeth = true from an unrelated path could silently reroute execution. Use pub(crate) or a dedicated setter.

Should fix — SystemTxInvalidCallee mapped to CallNotAllowedInsideStatic

An invalid callee in a system transaction has nothing to do with static calls. Users who see CallNotAllowedInsideStatic in a MegaETH test failure will search for a staticcall that does not exist. At minimum add a comment documenting this lossy approximation.

[claude]

All four issues from the previous review have been addressed:

• MegaCtx duplicate — single definition in evm.rs, properly imported in stack.rs. ✓
• Backend::is_megaeth visibility — now pub(crate) with a dedicated set_megaeth setter. ✓
• SystemTxInvalidCallee mapping — changed to NotActivated with an explanatory comment. ✓
• Alloy pin justification — PR description Note section explicitly attributes the exact-pin to mega-evm 1.5.1's dependency requirements. ✓

One new minor issue noted inline on cow.rs.

[claude]

LGTM. All feedback from both prior review rounds has been addressed:

• Third commit ("route MegaETH inspect dispatch through executor") cleanly resolves the cow.rs inline: the is_megaeth routing now lives exclusively in Executor::call_with_env / transact_with_env, keeping CowBackend::inspect and CowBackend::inspect_mega as dumb call-throughs with no branching logic.
• The four issues from the first review (MegaCtx duplicate, is_megaeth visibility, SystemTxInvalidCallee mapping, alloy pin justification) remain fixed.
• Validation layering (CLI → builder → inline-config) is airtight.

No further concerns.