feat: wire across to scroll and megaeth

[gomesalexandre]

Description

Across Protocol supports Scroll (534352) and MegaETH (4326) which we already have as chains but haven't wired in our Across swapper mapping. Also supports Solana now but that needs separate investigation.

Chains to wire

Chain	Our ChainId	Across ChainId
Scroll	eip155:534352	534352
MegaETH	eip155:4326	4326

Implementation details

Add to chainIdToAcrossChainId in packages/swapper/src/swappers/AcrossSwapper/constant.ts:

import { scrollChainId, megaEthChainId } from '@shapeshiftoss/caip'

// Add to chainIdToAcrossChainId:
[scrollChainId]: 534352,
[megaEthChainId]: 4326,

Entries go at the END of the object (append-only convention).

Verification

After wiring, verify quotes return from Across API for cross-chain swaps to/from these chains. Check the Across suggested-fees endpoint.

Risk

Low - config changes behind existing feature flags.

Agent prompt

Wire Scroll and MegaETH to the Across swapper. Edit packages/swapper/src/swappers/AcrossSwapper/constant.ts - import scrollChainId and megaEthChainId from @shapeshiftoss/caip, add entries to chainIdToAcrossChainId at the END of the object. Run pnpm run lint --fix and pnpm run type-check after.