feat: Remotion explainer video — realistic UI mockups (~60–90s) #115

Follow-up to #114. Reuse the scaffolding (RetentionNumber, LineageTag, Branding, Caption, timing.ts, theme/tokens.ts) but replace abstract stage visuals with realistic mockups: actual SIS-table styling, preflight validator UI, PDP submission console, AR file pull dialog, real Bishop State dashboard cell.

Extended runtime ~60–90s with sub-scenes for each governance callout (FERPA, sensitive-population safeguards, lineage view, definition tooltip).

Depends on: #114 (scaffold and shared components)
Spec status: to be brainstormed separately when #114 is merged

Acceptance criteria will be defined in the dedicated spec.