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/reversal_5d_all_universe_pipeline_report.md

@@ -30,8 +30,9 @@ costless research cumulative return at Sharpe
 54.31% daily hit rate.
 
 The binding constraint is **cost, not signal**: at ~148×/year
-turnover, a 10 bps round-trip (5 bps commission + 5 bps slippage) erases the
-edge — every variant is negative after costs. A tradable 5-day reversal needs
+turnover, a 10 bps one-way per-trade cost (5 bps commission + 5 bps slippage,
+charged on each leg — so ~20 bps per round trip) erases the edge — every variant
+is negative after costs. A tradable 5-day reversal needs
 turnover control, not a different signal.
 
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