fix(evm): make EnrichedMegaTx's cached tx_size/da_size reachable

[RealiCZ]

Summary

EnrichedMegaTx<T>'s MegaTransactionExt override for tx_size/estimated_da_size could never dispatch — the trait methods carry a where Self: Encodable2718 bound that EnrichedMegaTx never satisfied, so every caller silently recomputed from the raw inner transaction instead of reusing the precomputed fields.
This defeated the entire purpose of the wrapper: mega-reth's sequencer hot path already precomputes da_size/tx_size (often from a cheap mempool-cached value) and wraps it in EnrichedMegaTx expecting it to be reused, but MegaBlockExecutor::run_transaction ignored that and recomputed every time.
Closes #324.

Fix:

• Add impl Encodable2718 for EnrichedMegaTx<T> (delegating to the inner transaction) so the bound is satisfiable.
• run_transaction's existing signature is untouched — it's shared with the standard BlockExecutor::execute_transaction_with_commit_condition path, which is constrained by alloy_evm's ExecutableTx and can't require MegaTransactionExt.
• Add a new opt-in run_transaction_enriched / execute_mega_transaction_enriched entry point that reads tx_size/da_size from the outer Tx (hitting EnrichedMegaTx's cache) instead of unwrapping to the raw inner transaction.
• Benchmarked the isolated call cost: recompute is ~410-780ns depending on calldata size; the fixed path is ~0.36-0.37ns, matching a raw field read (crates/mega-evm/benches/enriched_tx.rs).

Trust boundary hardening:

• run_transaction_enriched feeds Tx-reported tx_size/da_size directly into pre_execution_check's consensus-relevant limit checks with no validation, and EnrichedMegaTx::new is a public constructor that accepts arbitrary values. Added a debug_assert cross-check against a fresh recompute (compiled out in release, so it costs nothing in production) and documented the caller contract on both run_transaction_enriched and EnrichedMegaTx::new.
• This is safe for the current single-sequencer architecture (the cache is always computed by the same trusted process); the doc comments call out that a future validation-path use of this method would need a different guarantee.

Test plan

• New unit test asserts EnrichedMegaTx's trait-dispatched tx_size/estimated_da_size/tx_hash return the stored fields, not a recomputed value.
• New MegaBlockExecutor integration test proves the fix changes real behavior: with a tx_da_size_limit set between a deliberately understated stored da_size and the real recomputed value, the same transaction is rejected under run_transaction and — before the debug_assert was added — was accepted under run_transaction_enriched.
• A second test (#[cfg(debug_assertions)], #[should_panic]) proves the debug_assert safety net actually catches that exact mismatch in debug/test builds. Verified it correctly does not run under cargo test --release (the assert compiles out there by design — documented as the intended profile-dependent trade-off, not an oversight).
• Ran the diff-scoped cargo mutants --in-diff gate locally against the full change: 0 missed mutants.
• Full cargo test -p mega-evm (debug and --release) and cargo test -p mega-evme pass.
• cargo fmt --all --check, cargo clippy --workspace --lib --examples --tests --benches --all-features --locked (debug and --release, checked for unused-import regressions from the new cfg(debug_assertions) gating), cargo sort --check, cargo doc -p mega-evm --no-deps (no new warnings), and the no_std/riscv64 target check all pass.
• cargo bench -p mega-evm --bench enriched_tx run locally to verify the new benchmark actually executes (not just compiles).

[github-actions]

🧬 Mutation testing — ✅ PASS

Nothing to test — no mutants were generated on the changed lines.

[codecov]

Codecov Report

❌ Patch coverage is 97.72727% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.6%. Comparing base (5a2ea61) to head (446020e).

Files with missing lines	Patch %	Lines
crates/mega-evm/src/block/helpers.rs	66.6%	1 Missing ⚠️

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[claude]

Label check

The spec:stable and api:compatible labels look correct now.

Missing comp:misc
mutants/suppressions.toml lives in the mutants/ directory at the repo root, outside all component crates, so comp:misc should be added alongside comp:core to accurately reflect that this PR touches non-core repo infrastructure.

[github-actions]

🧬 Mutation testing — ✅ PASS

Diff mutation score: 100.0% (1/1 viable mutants killed)

• caught: 1
• survived (real gaps): 0
• timed out (inconclusive): 0
• suppressed (equivalent/dead-code): 0
• unviable: 16 · timeout total: 0

No new test gaps introduced by this change. 🎉

[claude]

⚠️ 1 finding

• Summary: focused re-review of the EnrichedMegaTx cache-reachability fix, the new run_transaction_enriched entry point, and its test/bench surface.
• Nice: the debug_assert cross-check on run_transaction_enriched closes the trust-boundary risk of EnrichedMegaTx::new accepting arbitrary sizes without paying anything in release, and the #[cfg(debug_assertions)] #[should_panic] test proves that safety net actually fires on a mismatch — profile-dependent trade-off is documented on the method rather than left as a footgun.
• The run_transaction / run_transaction_enriched / run_transaction_with_sizes split cleanly separates the two size-resolution paths without changing the existing BlockExecutor-backed signature, and the mutation-test refresh replaces the suppressed-as-dead entry with a real assertion that the stored fields are what dispatches.
• Inline comments: 1 anchorable finding on the new Encodable2718 for EnrichedMegaTx<T> impl (type_flag delegation is incomplete).
• No still-open prior review threads.

[RealiCZ]

Thanks for the re-review. Addressed the inline type_flag finding directly on the thread (verified it wasn't actually mis-framing today, applied the suggested delegation anyway for consistency with this file's other trait impls — see 446020e).

Also went back and closed the codecov patch-coverage gap (14 lines, 67.4%) that was still open at that point: execute_mega_transaction_enriched had no test caller at all, and run_transaction_enriched's only test deliberately panics via its debug_assert before ever reaching the success path — so there was no happy-path test proving the enriched path actually succeeds and returns the cached values when they're accurate. Added one (446020e), reran scripts/coverage_mega_evm.sh locally to confirm all 14 lines are now covered.

comp:misc label added per the earlier bot comment — mutants/suppressions.toml is repo infra outside any crate, so that was a fair catch.

[claude]

Clean

• Reviewed the EnrichedMegaTx cache-plumbing fix: new Encodable2718 for EnrichedMegaTx impl to satisfy the trait bound, additive run_transaction_enriched / execute_mega_transaction_enriched entry points that read cached sizes, debug_assert cross-check against a fresh recompute, and matching tests + benchmark.
• run_transaction (shared with BlockExecutor::execute_transaction_with_commit_condition) is left untouched, so existing callers keep the recomputing behavior — the new path is opt-in.
• Trust-boundary risk (public EnrichedMegaTx::new + no release-mode revalidation before consensus-relevant limit checks) is called out explicitly in the doc comments on both EnrichedMegaTx::new and run_transaction_enriched, and the debug_assert gives a debug/CI safety net at zero release cost.
• Tests: the debug-only should_panic test proves the safety net actually fires on a mismatched cache, and the release-safe happy-path test proves execute_mega_transaction_enriched returns an outcome carrying the cached values.
• No new actionable findings.
• Still open from earlier reviews: none — the prior type_flag delegation thread was addressed in 446020e and has now been resolved.