feat: migrate SDK from ethers.js to viem, modernize toolchain

[janek26]

Summary

Replaces ethers.js with viem and modernizes the entire SDK toolchain. This is a breaking change for consumers.

viem migration

• All contract interactions use typed ABIs via parseAbi — zero as any casts in the codebase
• signPermit now uses walletClient.signTypedData() instead of raw private key signing via @metamask/eth-sig-util — works with any wallet type (browser, hardware, etc.)
• Properly typed QuoteExecutionDetails, PermitSignature, CrosschainQuoteExecutionDetails, and all public API functions
• Removed dependencies: @ethersproject/*, @ethereumjs/util, @metamask/eth-sig-util

Toolchain modernization

• Build: tsdx → zile (ESM-only, nodenext module resolution, TypeScript 5.5)
• Test: Jest (bundled in tsdx) → vitest
• Lint: ESLint + eslint-config-rainbow + 4 plugins → oxlint
• Format: Prettier → oxfmt
• tsconfig: Split into tsconfig.json (IDE + typecheck, includes tests) and tsconfig.build.json (zile build, src only)

CI

• SDK job now runs: typecheck → lint → format:check → build → test

Docs

• README.md updated with current API examples and dev commands
• MIGRATION.md added with full before/after guide for consumers migrating from ethers

Breaking changes

Before (ethers)	After (viem)
BigNumberish	BigIntish (string | number | bigint)
Signer / Wallet	WalletClient
StaticJsonRpcProvider	PublicClient
Transaction return	Hash (tx hash)
signPermit(publicClient, privateKey, ...)	signPermit(publicClient, walletClient, ...)
getWrappedAssetMethod(name, ...)	getWrappedAssetMethod('deposit' | 'withdraw', ...)
method(...methodArgs)	method() (args pre-bound)

Test plan

• yarn typecheck passes (src + tests)
• yarn lint passes (1 pre-existing warning: duplicate enum value)
• yarn format:check passes
• yarn build succeeds
• yarn test — 10/10 tests pass
• Verify consumer integration with wagmi/RainbowKit

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