Add Supertrend and VWAP strategies plus a short-side example

- New indicators: vwap (rolling/cumulative) and supertrend (line + direction),
  exposed on StrategyContext.
- New strategies: supertrend (flip-based, optional shorts), vwap_reversion
  (mean reversion below a rolling VWAP) and ema_cross_short (short-side
  example, mirroring the long EMA cross). 10 strategies total.
- Adds indicator and strategy tests; smoke-tested all three on live BTC/USDT
  5m data.

README.md

@@ -207,7 +207,8 @@ appears in the UI immediately. See **[AGENTS.md](AGENTS.md)** for the full
 template and the `StrategyContext` API.
 
 Bundled strategies: `sma_cross`, `ema_cross`, `macd_trend`,
-`bollinger_reversion`, `donchian_breakout`, `momentum`, `rsi_reversion`.
+`bollinger_reversion`, `donchian_breakout`, `momentum`, `rsi_reversion`,
+`supertrend`, `vwap_reversion`, `ema_cross_short` (short-side example).
 
 ## API