feat: add broker equity accessor

[stefan-jansen]

Summary

• add Broker.equity() as the preferred public account-equity accessor
• keep get_account_value() as an existing compatibility spelling that delegates to equity()
• document the accessor and add broker coverage

Validation

• uv run ruff check .
• uv run ruff format --check .
• uv run ty check
• pre-commit run --all-files
• uv run pytest tests/ -q
• uv build

[Copilot]

Pull request overview

Adds a new preferred public accessor (Broker.equity()) for marked account equity while keeping the existing get_account_value() method as a backward-compatible alias, and updates documentation/tests to reflect the new API surface.

Changes:

• Introduce Broker.equity() as the primary account-equity accessor and delegate get_account_value() to it.
• Add test coverage ensuring equity reflects current mark prices and that get_account_value() matches equity().
• Update user guide/API docs to document the new accessor and clarify compatibility behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/test_broker.py	Adds a test verifying equity() uses current mark prices and matches get_account_value().
src/ml4t/backtest/broker.py	Implements equity() and makes get_account_value() delegate to it.
docs/user-guide/stateful-strategies.md	Documents equity() as the preferred accessor and notes get_account_value() compatibility.
docs/api/index.md	Exposes equity in the generated Broker API members list.

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