fix: correct short position return calculation, bump to v0.1.6

Short trades were incorrectly using long formula (exit/entry - 1).
Now correctly uses (2 - exit/entry - 1) for short positions based
on position_weight sign.

- Modified WideTradeRecord::calculate_return()
- Modified TradeRecord::calculate_return()
- Added unit tests for long/short return calculations

Author: Yvictor

.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "polars-backtest",
-  "version": "0.1.5",
+  "version": "0.1.6",
   "description": "Polars backtest extension development helper - backtesting with polars expressions",
   "author": {
     "name": "Yvictor",