BRK: Apply spread at trade entry and exit

[nyxst4ck]

Closes #1335.

Summary

• apply the configured bid-ask spread adversely at both trade entry and trade exit
• retain the existing commission model, including recalculation for each transaction
• document the two-sided spread behavior
• cover long/short round trips, reversals, stop-loss fills, and updated equity expectations

This revision follows the maintainer feedback: the spread is no longer split in half, and the commission accounting changes from the earlier draft have been removed.

Validation

• python -m backtesting.test — 81 passed, 1 skipped
• git diff --check

[kernc]

@nyxst4ck Thanks for picking this up!

1. split the configured bid-ask spread equally between trade entry and exit

Modelling spread as a share of price is somewhat eccentric and variable already. But then "you need to further compute halves" will be a documentation nightmare, leaving the logic all but simple. Spread is property of the instrument and it affects us, in whole, when entering the trade.

Then it affects us again when we try to sell the gold! Thus, to be precise, I'm thinking spread should also be accounted for twice. 🤔😳

But this shall break a lot of existing expected values.

But breaking spread in half is a decisive no-go. 👎

3. preserve the commission actually charged at entry instead of recomputing it at close

This is also not how exchanges do. The commission, for what I see, is computed again based on the order value at the time of the transaction.

4. allocate entry commission proportionally when a trade is partially closed

Fixing 3. proper makes this a no-op.

[nyxst4ck]

Makes sense, thanks for the detailed explanation. Reworked: spread is now applied in full at both entry and exit (_adjusted_price no longer halves it), and the proportional entry-commission allocation on partial closes is gone — commission is simply recomputed per transaction, same as before. python -m backtesting.test: 81 passed, 1 skipped.