feat: metric definitions glossary with IPEDS and state-compliance cross-walks

[William-Hill]

Context

From AASCU Intermediary feedback session (see docs/aascu_intermediary_feedback_summary.md, pain point B):

"There's something for completion rates at three years, four years, five years, but they are not very much clear... they also reflect to the documentation."

"The definitions are completely different from what they're used to reporting for state compliance, for IPEDS, for anything else that they're reporting on — then why on earth would they invest any time in learning this new tool?"

Institutional Research staff already report on the same students under multiple regimes (PDP, IPEDS, state compliance). Forcing them to mentally re-map every term is a primary adoption barrier.

Goal

Every metric or KPI displayed in the dashboard surfaces its definition in-context, with explicit cross-walks to IPEDS and state-compliance equivalents where applicable.

Scope

• Hover tooltip on every KPI / chart axis / table column showing:
  • Plain-English definition
  • PDP definition (if relevant)
  • IPEDS-equivalent term (if any)
  • State-compliance term (if any)
  • Link to deeper /glossary entry
• New /glossary page indexed alphabetically + by topic (retention, completion, persistence, etc.)
• Definitions stored as versioned markdown alongside the code (single source of truth)
• Reuse existing components/info-popover.tsx as the tooltip surface

Out of scope

• Backfilling definitions into the existing methodology/ page (link from glossary instead)

Acceptance criteria

• Every KPI card on the dashboard shows a tooltip with at least PDP + plain-English definition
• /glossary page lists all metrics with full cross-walks
• Definitions live in a markdown file checked into the repo
• No metric appears in the UI without a corresponding glossary entry (lint check)