docs: clarify same-bar close fill semantics

[tam7777]

Summary

• document that signal rows currently rebalance on the same bar using that bar's close
• add the same note to the API reference
• add a matching inline comment in the engine loop

Verification

• pytest tests/test_core.py -q

Notes

• broader optional concordance tests require extra engine dependencies (for example ) that are not provisioned in this environment