fix(polymarket): raise default backtest ROI posture

[taariq]

Summary

• raise default Polymarket backtest configs from conservative demo settings to a more promotional $1,000 bankroll posture
• align script fallbacks with the example configs so the runtime does not silently revert to low-notional defaults
• add regression tests that assert the default config posture clears 20% return in representative synthetic backtests

Representative synthetic backtest results

• maker rebate bot: 28.783%
• paired market basis maker: 22.7423%
• liquidity paired basis maker: 22.7423%
• high throughput paired basis maker: 32.7351%

Testing

• pytest -q polymarket/maker-rebate-bot/tests/test_smoke.py
• pytest -q polymarket/paired-market-basis-maker/tests/test_smoke.py
• pytest -q polymarket/liquidity-paired-basis-maker/tests/test_smoke.py
• pytest -q polymarket/high-throughput-paired-basis-maker/tests/test_smoke.py

Closes #72