feat: wire across to scroll and megaeth

[gomesalexandre]

Description

Does what it says on the box. Across supports Scroll (534352) and MegaETH (4326) which we already have as chains but weren't wired up.

Verified via Across available-routes API:

• Scroll: WETH bridge route confirmed
• MegaETH: 3 routes confirmed (WETH, native ETH, USDT)

closes #12140

Risk

Low - config changes behind existing feature flags.

Testing

Engineering

Verify type-check and lint pass. ACROSS_SUPPORTED_CHAIN_IDS now includes Scroll and MegaETH.

Operations

• 🏁 My feature is behind a flag and doesn't require operations testing (yet)

Summary by CodeRabbit

• New Features
  • Added support for Mega ETH and Scroll chains in the swapper, enabling users to execute swaps on these blockchain networks.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 488e9596-6898-4a9e-a71f-28dfc077bac8

📥 Commits

Reviewing files that changed from the base of the PR and between ead6a6f and 4cdb55a.

📒 Files selected for processing (1)

• packages/swapper/src/swappers/AcrossSwapper/constant.ts

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📝 Walkthrough

Walkthrough

Adds support for two new blockchain chains (Scroll and MegaETH) to the AcrossSwapper by importing their chain identifiers and extending the chainIdToAcrossChainId mapping with corresponding Across protocol chain identifiers.

Changes

Cohort / File(s)	Summary
AcrossSwapper Chain Support packages/swapper/src/swappers/AcrossSwapper/constant.ts	Imports scrollChainId and megaEthChainId from caip, imports scroll from viem/chains, and adds two new chain mappings to chainIdToAcrossChainId: scrollChainId→534352 and megaEthChainId→4326.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

Across the chains we hop with glee,
Scroll and MegaETH, wild and free!
New mappings added, simple and neat,
Config changes make swaps complete. 🐇✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'feat: wire across to scroll and megaeth' clearly and concisely summarizes the main change of adding Scroll and MegaETH chain support to the Across swapper mapping.
Linked Issues check	✅ Passed	The pull request implementation aligns with all coding requirements from issue #12140: imports scrollChainId and megaEthChainId from caip, adds mappings to chainIdToAcrossChainId with correct values (scrollChainId -> 534352, megaEthChainId -> 4326), and follows the append-only convention.
Out of Scope Changes check	✅ Passed	All changes are directly related to the linked issue objectives of wiring Scroll and MegaETH into the Across swapper mapping with no extraneous modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/across-more-chains

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[gomesalexandre]

QA Verification - Across quotes on localhost

Ran local dev server on this branch and verified Across returns quotes for Scroll.

Test: ETH -> SCR on Scroll via Across

• Pay With: 0.01 ETH (~$20.73)
• You Get: 496.45 SCR (~$20.78, +0.23%)
• Protocol: Across (confirmed in the UI)
• Rate: 1 ETH = 49,644.93 SCR
• Fee: $0.120

Scroll chain is properly wired and Across is returning quotes. MegaETH couldn't be tested via UI (limited token pairs) but the route was verified via Across available-routes API (3 routes: WETH, native ETH, USDT).

[NeOMakinG]

✅ Code Review — LGTM

Changes: Add Scroll and MegaETH to Across swapper chain mapping.

Verification:

• Chain IDs match Across available-routes API (scroll.id = 534352, MegaETH custom ID 4326)
• Simple mapping addition, no logic changes
• CI passes ✅

Clean config addition.