Cross-cohort QC comparison dashboard

[amc-corey-cox]

Build an interactive view of QC results across all cohorts, building on the per-cohort reports from the QC reporting tool (#303).

Features

• Cross-cohort grade matrix (variable × cohort, like the current tabular summary)
• Drill-down from grade to per-variable detail
• Coverage report — which variables are present/absent per cohort (T/B matrix)
• Filter by domain (Demographics, Anthropometrics, Blood Pressure, etc.)

Context

This is the visualization layer on top of the machine-readable scores produced by the QC reporting tool. Current presentation is monospaced text tables in slide decks — this makes it interactive and explorable.

Depends on #303 being in place first.