fix: resolve backtest review cleanup items

[stefan-jansen]

Summary

• apply contract multipliers to percentage margin validation and buying power checks
• classify integer-share orders after rounding across execution paths
• validate and safely serialize percentage margin schedules
• clarify the legacy account value alias

Verification

• uv run ruff check src tests
• uv run ty check
• uv run pytest tests/ -q
• pre-commit run --all-files

[Copilot]

Pull request overview

This PR addresses several backtest “review cleanup” correctness items across margin accounting, order execution, and configuration serialization, improving parity and safety for futures/margin workflows.

Changes:

• Apply contract multipliers to percentage-based margin validation and buying power calculations (including prechecks and gatekeeper validation paths).
• Ensure integer-share rounding occurs before exit/entry classification across execution paths (notably EXIT_FIRST and sequential processing).
• Harden config validation and YAML serialization for margin percentage schedules (including safe YAML dumping and new tests).

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test_config_wiring.py	Adds regression tests for integer-share rounding classification and margin_pct_schedule validation/YAML roundtrip.
tests/accounting/test_margin_account_policy.py	Adds futures multiplier coverage for percentage margin requirements and buying power.
src/ml4t/backtest/core/order_book.py	Threads contract multipliers into NEXT_BAR submission precheck policy calls.
src/ml4t/backtest/core/execution_engine.py	Moves integer-share rounding earlier to ensure correct exit/entry classification and consistent rejection behavior.
src/ml4t/backtest/config.py	Adds margin schedule coercion/validation helpers and switches YAML writing to safe_dump.
src/ml4t/backtest/broker.py	Passes multiplier resolver into gatekeeper and clarifies get_account_value() as an equity() alias.
src/ml4t/backtest/accounting/policy.py	Applies multipliers to percentage margin, buying power, and cash/margin checks; validates margin_pct_schedule on policy construction.
src/ml4t/backtest/accounting/gatekeeper.py	Adds multiplier plumbing into policy validation calls via a resolver callback.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.