fix: preserve integer affordability boundary

[stefan-jansen]

Summary

• use integer-domain bisection for integer partial fills so exact affordability boundaries are not rounded down by float truncation
• add a regression test for an exact cash boundary

Verification

• uv run pytest tests/test_config_wiring.py::TestPartialFills -q
• pre-commit run --all-files
• uv run ty check
• uv run pytest tests/ -q
• uv run mkdocs build --strict

Addresses Copilot thread PRRT_kwDOQWVsd86QIx3N from PR #36.

[Copilot]

Pull request overview

This PR improves the backtesting engine’s partial-fill sizing for ShareType.INTEGER by switching affordability sizing from float-domain bisection to integer-domain bisection, preventing boundary underfills caused by float truncation. It also adds a regression test to lock in the exact-cash boundary behavior.

Changes:

• Implement integer-domain bisection in FillEngine.get_max_affordable_quantity() when share_type == INTEGER.
• Add a regression test asserting an exact affordability boundary fill results in zero remaining cash.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/test_config_wiring.py	Adds regression coverage for exact cash boundary behavior with integer partial fills.
src/ml4t/backtest/core/fill_engine.py	Uses integer-domain bisection for integer-share affordability sizing to avoid float truncation underfills.

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