Flag dead action/route references in dashboard header actions (ADR-0049 for references) #3367

Summary

Dashboard header actions[] (and widget actionUrl) can reference actions or routes that do not exist. Nothing in os validate / os build flags them, so they ship as buttons that silently do nothing when clicked. This is the ADR-0049 "enforce-or-remove" principle applied to references rather than to properties.

Surfaced while reviewing a production app on 16.0.0-rc.1.

Repro

Dashboard header actions such as:

header: {
  actions: [
    { label: 'Export PDF', actionType: 'script', actionUrl: 'export_dashboard_pdf' },
    { label: 'New Deal',   actionType: 'modal',  actionUrl: 'create_opportunity' },
    { label: 'Forecast',   actionType: 'url',    actionUrl: '/reports/forecast' },
  ],
}

• export_dashboard_pdf / create_opportunity are script / modal targets but no such action is defined anywhere in the stack.
• /reports/forecast is a url target that matches no registered report/view/object route.

All of these pass os validate and os build; at runtime the buttons render and do nothing. Authoring a non-existent target is a false affordance — the exact failure mode ADR-0049 exists to prevent for metadata properties.

Proposed check

Add reference-integrity validation for action targets:

• actionType: 'script' | 'modal' → error/warn unless actionUrl resolves to a defined action.
• actionType: 'url' → warn unless the path resolves to a registered view / report / object route (in-app route shape).

Applies to dashboard header.actions[], widget actionUrl, and any other surface that references an action/route by id or path.

Acceptance criteria

• A dashboard action referencing an undefined script/modal action fails validation with a message naming the dashboard, button label, and unresolved target.
• A url action pointing at a non-existent in-app route is at least warned.
• Valid references (defined action / real route) pass.
• Fixture coverage for defined vs dangling targets.

Lower priority than the two runtime-crash cases, but closes the same "declared ≠ enforced" gap for references.

Related shift-left checks from the same 14.7 → 16 upgrade: #3365, #3366, #3367.