feat: scope multisource daily assurance by input field

[Pigbibi]

Purpose

Allow a research-input contract to declare exactly which daily price fields and whether volume are required for independent source agreement. The field scope is embedded in the immutable policy digest.

Safety

• Defaults remain strict and backward-compatible: all OHLC fields plus volume are still compared.
• A caller must explicitly declare a narrower field set; there is no silent tolerance increase or source fallback.
• This supports candidates whose downstream P3 consumes only a documented subset of fields, without treating irrelevant vendor rounding as full-input agreement.

Validation

• uv run --no-sync pytest -q tests/test_multisource_assurance.py (10 passed)
• uv run --no-sync ruff check src/quant_platform_kit/data/multisource_assurance.py tests/test_multisource_assurance.py (passed)