[WIP] Stabilize date_col convention by making it a class-level attribute

[Copilot]

• Explore repository and understand current date_col usage
• Run baseline tests (344 pass)
• Update _data.py: replace date_col property with a proper dataclass field date_col: str = "Date", and thread date_col=self.date_col through all internal Data() construction calls
• Update api.py: change build_data() default to Data.date_col and pass date_col=date_col to Data(...) constructor
• Update _stats.py: fix all self.data.date_col usages (was list[str], now str), and pass date_col=self.data.date_col when constructing Data objects internally
• Update test in test_data.py: date_col now returns str, not list[str]
• Run tests and verify all pass
• Code review + CodeQL check

Original prompt

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This section details on the original issue you should resolve

<issue_title>Stabilize date_col convention — Consider making date_col a class-level attribute on Data instead of threading it through function parameters</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Stabilize date_col convention — Consider making date_col a class-level attribute on Data instead of threading it through function parameters #399

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