Align remaining documentation and provenance

[devin-ai-integration]

Summary

• Keep data_source.txt provenance to the provider and plan, leaving data age to last_trading_day.
• Remove remaining unconditional delay, retention, count, and chart-panel assumptions from current documentation.
• Correct the withdrawn ref_index.csv wording and update the contract tests.

Validation

• pytest
• ruff check .
• sh -n run.sh
• git diff --check

Link to Devin session: https://app.devin.ai/sessions/a734c9c2909a411a80293287c93606be
Requested by: @id774

[devin-ai-integration]

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