fix: align L2 header fields between assemble and import paths

[panos-xyz]

Summary

Fixes a block hash mismatch between the assembleL2Block (assemble path) and newL2Block (import path) by ensuring both sides construct identical headers.

Root Causes

1. mix_hash: MorphBlockAssembler used prevrandao() which can be non-zero, while the import side used B256::ZERO. Morph L2 always keeps mix_hash = 0 (confirmed from geth source).

2. withdrawals_root / blob_gas_used / excess_blob_gas: Import side conditionally set these based on Shanghai/Cancun hardfork activation, but MorphBlockAssembler always emits None for all three (Morph L2 has no withdrawals or blob transactions). This caused a hash divergence on any network where Shanghai/Cancun is active.

Changes

• crates/evm/src/assemble.rs: mix_hash: Default::default() (was prevrandao())
• crates/engine-api/src/builder.rs:
  • withdrawals_root, blob_gas_used, excess_blob_gas → always None
  • Extract header_and_body_from_executable_data() helper
  • Remove unused EMPTY_WITHDRAWALS and EthereumHardforks imports

Test plan

• cargo check -p morph-engine-api -p morph-evm passes
• Existing unit tests pass
• New block import via newL2Block no longer returns hash mismatch errors

Summary by CodeRabbit

• Chores
  • Removed hardfork dependency and related code to streamline dependencies
  • Refactored block header and body assembly logic
  • Updated header field handling for consistent L2 block structure

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 74709ff2-e6c8-4951-94b5-46924ebb7a05

📥 Commits

Reviewing files that changed from the base of the PR and between b2275d4 and 8a7278c.

⛔ Files ignored due to path filters (1)

• Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (3)

• crates/engine-api/Cargo.toml
• crates/engine-api/src/builder.rs
• crates/evm/src/assemble.rs

💤 Files with no reviewable changes (1)

• crates/engine-api/Cargo.toml

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📝 Walkthrough

Walkthrough

The PR removes the alloy-hardforks dependency and refactors header/body construction in the engine-api builder to eliminate conditional logic based on hardfork timestamps. It also simplifies EVM block assembly by setting mix_hash to a constant zero value instead of deriving it from prevrandao.

Changes

Cohort / File(s)	Summary
Dependency Cleanup crates/engine-api/Cargo.toml	Removed the unused alloy-hardforks workspace dependency from the engine-api crate.
Builder Refactoring crates/engine-api/src/builder.rs	Extracted header and body construction into a new helper function header_and_body_from_executable_data(); removed hardfork timestamp-based conditionals (shanghai_active, cancun_active); made withdrawals_root, blob_gas_used, and excess_blob_gas unconditional fields; added PartialEq derive to InMemoryHead.
EVM Assembly crates/evm/src/assemble.rs	Changed mix_hash to always default to zero instead of deriving from evm_env.block_env.prevrandao().

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• refactor(engine-api): simplify engine API and fix code quality issues #35: Modifies the same engine-api and evm files, affecting header/payload construction and engine API wiring.
• feat: add morph engine api #12: Added crates/engine-api with hardfork-related dependencies that are now being removed in this PR.

Suggested reviewers

• anylots
• chengwenxi

Poem

🐰 A rabbit hops through Morph's code with glee,
Hardforks untangled, L2-only and free!
No Shanghai, no Cancun conditions to weigh—
Just clean, simple logic to brighten the day! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the main change: aligning L2 header fields between the assemble and import code paths to resolve block hash mismatches.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/l2-header-hash-alignment

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[anylots]

It is recommended to add test coverage: ensure that corresponding unit tests and integration tests are included.

[panos-xyz]

It is recommended to add test coverage: ensure that corresponding unit tests and integration tests are included.

Yeah, these issues were caught by the e2e tests in the subsequent PR, and there will be corresponding tests to cover them.