feat: configurable entity column in DataFeed

[stefan-jansen]

Summary

• DataFeed now auto-detects the entity identifier column from prices/signals data
• Detection order: symbol > asset > product > ticker
• New entity_col keyword parameter for explicit override
• No more hardcoded "asset" column name — existing code with asset column still works

Why

The book codebase uses symbol as the canonical entity column (and product for CME futures). DataFeed hardcoded asset, requiring callers to rename columns before passing data. This was fragile and got lost on every uv sync.

Test plan

• All existing DataFeed tests pass (backward compat with asset column)
• New tests: auto-detect symbol, asset, product; explicit override; error cases; signals with symbol

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR makes the DataFeed class's entity identifier column configurable instead of hardcoding "asset". It adds auto-detection (checking symbol > asset > product > ticker in order) and an explicit entity_col keyword parameter for override.

Changes:

• Added ENTITY_COL_CANDIDATES class constant and _resolve_entity_col classmethod to auto-detect or validate the entity column
• Added entity_col keyword-only parameter to DataFeed.__init__ and replaced all hardcoded "asset" references with the resolved _entity_col
• Added comprehensive tests covering auto-detection, explicit override, error cases, and signals integration

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/ml4t/backtest/datafeed.py	Add entity column auto-detection and entity_col parameter; replace hardcoded "asset" with dynamic _entity_col
tests/test_datafeed_memory.py	Add TestDataFeedEntityColumn test class covering auto-detect, priority, explicit override, and error cases

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