Document cost bps as one-way per-trade, not round-trip

The simulator charges (cost_bps + slippage_bps) on each fill, so a full
round trip is charged twice. Correct the cost-model doc, the reversal_5d
report, and the report generator to state the rate is one-way per-trade
(~20 bps round trip for 5+5), rather than mislabeling it round-trip.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/portfolio_trading_cost_model.md

@@ -41,6 +41,13 @@ execution_price_i = open_price_i
 rates are added linearly. The CLI options `--cost-bps` and `--slippage-bps`
 both default to `0.0`.
 
+Both rates are **one-way, per-trade**: the combined `(cost_bps + slippage_bps)`
+is charged on the traded notional of *each* fill, buy and sell alike. A full
+round trip (enter then exit a position) is therefore charged twice — e.g.
+`5 + 5` bps becomes ~20 bps over a complete round trip, not 10. Quote any
+round-trip figure by doubling, or convert a round-trip budget to a per-trade
+rate by halving before passing it in.
+
 Example:
 
 ```text