Exclude dividends from investment returns without blocking known purchase cost #161

Problem

Dividend events currently invalidate known purchase cost and price gains and leave a misleading purchase-review warning after historical payment details are saved.

Steps to reproduce

1. Use a reconciled stock holding with confirmed purchase payment facts.
2. Load market history containing a dividend after the purchase.
3. Review and save purchase details; Needs attention remains because of unsupported_event.

Expected behavior

Investment returns exclude dividends explicitly. Dividend events alone do not invalidate units, known basis, cash flows, price gains or daily price returns. Users can record received dividends separately as ordinary cash income. Splits still require supported position history.

Acceptance criteria

• API and independent MCP use dividend-excluding current metrics, daily estimates and portfolio history consistently.
• Dividend events do not trigger purchase-review warnings; return views and read metadata clearly disclose exclusion.
• Split and missing-history safeguards remain in place, including pre-range splits.
• Regression tests cover dividends, splits and unchanged financial facts; verify the local restored export without resetting user edits.

Context or visuals

Child of #123. Follow-up to #159. No dividend posting, reinvestment, tax handling or schema changes are requested. Ordinary cash income stays separate from investment returns.