Add # Long Trades and # Short Trades to stats output

[wavebyrd]

Summary

Add two new statistics to the backtest results output:

• # Long Trades: count of trades with positive size (long positions)
• # Short Trades: count of trades with negative size (short positions)

Example output

# Trades                                   23
# Long Trades                              15
# Short Trades                              8
Win Rate [%]                         65.21739

Motivation

This helps users gauge how balanced their bull and bear models are without needing to manually filter the trades dataframe. As noted in the issue, this is useful for understanding strategy behavior.

Implementation

Simple addition in _stats.py - counts trades based on the sign of the Size column:

• Size > 0 = long trade
• Size < 0 = short trade

Test plan

• Existing tests pass (new stats computed alongside existing ones)
• Verify counts match manual filtering of trades dataframe

Fixes #1309

🤖 Generated with Claude Code

[wavebyrd]

Friendly ping on this feature addition. Let me know if you'd like any changes to the implementation!