feat(ts-sdk): add buildOrder method and BuiltOrder type

[Aboudjem]

Closes #53

Summary

• Adds buildOrder(params) to the TypeScript SDK Exchange class, mirroring the existing hand-maintained pattern used by createOrder, fetchOHLCV, etc.
• Adds BuiltOrder interface to pmxt/models.ts matching the core BuiltOrder type (already defined in core/src/types.ts and documented in API_REFERENCE.md)
• Adds buildOrder and submitOrder to the surface test's PUBLIC_METHODS list

How it works

buildOrder POSTs to /api/{exchange}/buildOrder on the sidecar, which runs the exchange's buildOrder implementation and returns the signed/unsigned payload without submitting. The caller gets back a BuiltOrder with exchange, params, signedOrder (CLOB), tx (AMM), and raw fields — ready to forward through middleware or sign independently.

const built = await exchange.buildOrder({
  marketId: market.marketId,
  outcomeId: market.yes.outcomeId,
  side: 'buy',
  type: 'limit',
  amount: 10,
  price: 0.55,
});
// built.signedOrder — EIP-712 payload for Polymarket CLOB
const order = await exchange.submitOrder(built);

Test plan

• buildOrder and submitOrder appear in PUBLIC_METHODS surface test (212/212 passing)
• node sdks/typescript/scripts/generate-client-methods.js produces 0 diff (CI drift check passes)
• Full TypeScript SDK test suite: 212/212 pass