Add the Williams %R strategy

New williams_r indicator (in [-100, 0]) exposed as ctx.williams_r(period), and a
long-only mean-reversion strategy: enter when %R is at/below the oversold level,
exit when it reaches the overbought level, with an optional ATR stop. 11
strategies total. Adds indicator and strategy tests; smoke-tested on live
BTC/USDT 5m (backtest + parameter sweep).

README.md

@@ -208,7 +208,8 @@ template and the `StrategyContext` API.
 
 Bundled strategies: `sma_cross`, `ema_cross`, `macd_trend`,
 `bollinger_reversion`, `donchian_breakout`, `momentum`, `rsi_reversion`,
-`supertrend`, `vwap_reversion`, `ema_cross_short` (short-side example).
+`supertrend`, `vwap_reversion`, `ema_cross_short` (short-side example),
+`williams_r`.
 
 ## API