[NF-20] [BACKEND] SDK: Governance Client

Overview

The TypeScript SDK only ever wrapped VaultRouter (its original GF-08 scope). This issue adds a first-class client for the Governance contract, which currently has zero SDK surface at all.

Problem

sdks/typescript/src/index.ts's YieldLadder class exposes exactly four methods: deposit, withdraw, earlyExit, position — all against VaultRouter. There is no way for a frontend or third-party integrator to call propose, veto, or execute on Governance, or to read a proposal's status, without hand-rolling raw Soroban RPC calls (which is exactly what the current app/src/app/governance/page.tsx ends up needing to do ad hoc, since no SDK method exists for it).

Proposed Solution

• Add a YieldLadder.governance sub-client (or a separate exported GovernanceClient class, consistent with whichever pattern the maintainers prefer) with:
  • propose(poolId: string, targetBps: number): Promise<number> (returns proposal id)
  • veto(proposalId: number): Promise<void>
  • execute(proposalId: number): Promise<void>
  • proposal(proposalId: number): Promise<Proposal> — new exported Proposal type matching the on-chain struct (id, action, proposedAtLedger, status)
  • pendingProposals(): Promise<Proposal[]>
• Resolves the Governance contract address from deployments/{network}.json, following the same address-resolution pattern already used for VaultRouter.
• Simulates before submitting for all state-changing calls, consistent with the existing deposit/withdraw/earlyExit pattern.

Acceptance Criteria

• All five methods above are implemented and exported
• proposal()/pendingProposals() correctly deserialize the on-chain Proposal struct into the new TypeScript Proposal type
• veto/execute correctly surface on-chain rejection reasons (timelock not elapsed, already vetoed, etc.) as typed errors, not raw RPC error strings
• Governance contract address is resolved per-network, matching the existing VaultRouter address-resolution pattern
• Unit tests cover: propose returns an id, veto/execute call the right contract method with the right args, proposal deserialization

Start in sdks/typescript/src/index.ts, reading the existing VaultRouter address resolution and deposit, withdraw, and earlyExit simulation patterns; compare the ad hoc calls in app/src/app/governance/page.tsx. Add the five Governance operations, Proposal deserialization, per-network address lookup, and typed rejection errors, then add unit tests covering the listed calls and deserialization.