Phase A of RSU tax spike fix. Two changes:
1. Panel 2 "Monthly cash flow (RSU stripped)" plotted raw income_tax despite
the title. Switch to COALESCE(cash_income_tax, income_tax) so the chart
is honest once the Phase B back-fill populates cash_income_tax on
variant-A slips. For slips where cash_income_tax is already populated
(variant B, 2024+) the spike is removed immediately.
2. Panel 4 "Data integrity" now surfaces rows where cash_income_tax is NULL
on vest months (rsu_vest > 0). New status value NULL_CASH_TAX (orange)
highlights the back-fill remaining population — expected to drop to 0
after Phase B lands.
Part of: code-860
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