fix(trading): include fees in cash risk checks

[ECD5A]

Summary

• add a pre-trade fee estimate to the ExchangeClient contract
• use the same fee calculation for risk checks and simulated fills
• reject buys when notional plus the estimated fee exceeds available cash
• add regression coverage at both the risk-engine and trading-engine boundaries

Receipt

Before this change, a portfolio with $100.00 cash could buy 0.001 BTC @ $100,000 with a 10 bps fee. The risk check compared only the $100.00 notional to cash and allowed the order; the fill then debited $100.10, leaving cash at -$0.10.

The two new regression tests reproduced that failure before the implementation change: the direct risk decision was allowed: true, and the engine outcome was filled. Both now block the order before it reaches the exchange.

Design alignment

This advances Conviction 5: the deterministic pre-trade guard is the last line of defense. It also protects the cash/NAV accounting invariant described in ADR 0005.

Tests

• npm run build
• node --test --test-reporter=spec --test-name-pattern=RiskEngine|TradingEngine|MockExchange|LiveExchange test/local.mjs — 14 passed
• git diff --check

A full local npm test run reaches the relevant trading suite successfully but has unrelated existing failures in Windows HOME/temp-directory isolation tests.