[codex] Add Fast MetaMask Snap SDK support

[seichris]

Summary

Rebases the Fast MetaMask Snap work onto the current monorepo layout and adds the SDK-side support needed for delegated browser signing.

What Changed

• add FastSnapClient and a new @fastxyz/sdk/browser entrypoint
• add a transport abstraction so FastProvider can use custom request adapters
• refactor transaction signing to work with delegated signers, not only raw private-key holders
• keep Signer compatible while adding the exported FastSigner interface
• add unit coverage for custom transports, delegated transaction signing, and Snap RPC flows
• update the SDK README and monorepo README to document browser Snap usage

Why

main moved to the monorepo and released @fastxyz/sdk 1.0.1, so the original branch no longer matched the published package layout. This rebases the Snap support onto the current package structure and clears the merge conflicts on the PR.

Validation

• pnpm --filter @fastxyz/sdk... build
• pnpm --filter @fastxyz/sdk test