feat(core): add TickData struct, TimeExit reason, compute_backtest_metrics pub fn

- TickData: parallel tick arrays (timestamps, ltp, bid, ask, buy_qty_delta,
  sell_qty_delta, oi) with len/is_empty helpers; callers must pre-convert
  Zerodha cumulative totals to per-tick deltas before passing
- ExitReason::TimeExit: max hold time exceeded variant for tick backtest
- compute_backtest_metrics: pub free fn wrapping PortfolioEngine::calculate_metrics
  so non-OHLCV strategies can produce identical metrics without duplication

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/portfolio/engine.rs

@@ -761,6 +761,25 @@ impl PortfolioEngine {
     }
 }
 
+/// Compute `BacktestMetrics` from pre-built curves and trade list.
+///
+/// Exposed as a standalone function so non-OHLCV strategies (e.g. tick backtest)
+/// can produce identical metrics without duplicating the calculation logic.
+pub fn compute_backtest_metrics(
+    equity_curve: &[f64],
+    drawdown_curve: &[f64],
+    returns: &[f64],
+    trades: &[Trade],
+    initial_capital: f64,
+) -> BacktestMetrics {
+    // Delegate to a throwaway engine instance — avoids duplicating the logic.
+    let engine = PortfolioEngine::new(BacktestConfig {
+        initial_capital,
+        ..Default::default()
+    });
+    engine.calculate_metrics(equity_curve, drawdown_curve, returns, trades, &StreamingMetrics::new())
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;