Add HOSE lot-aware position sizing helper

[toreleon]

What changed

• Added src/risk/positionSizing.ts with a pure calculatePositionSize helper for risk-based long sizing.
• The helper validates equity, risk fraction, entry/stop prices, and lot size with clear error messages.
• Added tests/positionSizing.test.ts covering the worked example, lot rounding, zero-share rounding, and all validation branches.

Verification

• pnpm install
• pnpm typecheck
• pnpm test

Both checks passed after the implementation.