fix: map cashValue to unrealizedPnL in polymarket_us

[realfishsam]

Fixes #533

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Maps the Polymarket US cashValue field from the position object to compute currentPrice and unrealizedPnL, replacing the previous hard-coded zeros with real mark-to-market values derived from cashValue - abs(cost).

Blast Radius

• Polymarket US exchange: core/src/exchanges/polymarket_us/normalizer.ts (position normalization only)
• No type changes, no OpenAPI changes, no SDK changes -- the unified Position type already has currentPrice and unrealizedPnL fields

Findings

1. Division-by-zero guard is present. size > 0 ? cashValue / size : 0 correctly guards the currentPrice calculation.

2. fromAmount(pos.cashValue) assumes cashValue exists on the SDK type. The polymarket-us SDK's UserPosition type may not declare cashValue in its TypeScript definitions. If it does not, this will need a cast or type extension. The code compiles if fromAmount accepts undefined gracefully (returning 0), but this should be verified. A grep for cashValue in the codebase returns zero results on the main branch, suggesting this is a new field being accessed for the first time.

3. PnL formula is correct. unrealizedPnL = cashValue - abs(cost) is the standard mark-to-market formula: current value minus entry cost.

4. Comment quality is good. The inline comments explain the derivation clearly.

PMXT Pipeline Check

• Field propagation: OK (Position type already has these fields, no new fields added)
• OpenAPI sync: N/A (no BaseExchange.ts change)
• Type safety: OK (assuming pos.cashValue exists at runtime)

Semver Impact

patch -- fills in previously-zero fields with real data, no API shape change

Risk

• pos.cashValue existence at runtime is unverified. If the Polymarket US SDK does not include cashValue on the position object, fromAmount(undefined) must return 0 for this to not crash. Test against a live account with open positions.
• For positions where cashValue is 0 or negative (fully underwater), verify the sign conventions produce correct PnL reporting.