[frontend-contrib S10] Scaffold template wiring + HostSurfaceDescriptor + vite feed

[rickylabs]

Part of #922

Scope

New-app templates wire the layer by default: generated registry import in main.ts, fresh({ islandSpecifiers }) in vite.config, PluginZone placements (topbar.end, dashboard.panels, home.cards, footer), nav feed merge, css import; the app's versioned HostSurfaceDescriptor ships in the template.

Design source: RFC #890 (rfc.md §5) + .llm/runs/plan-frontend-contrib--seed/design/canonical/ (rev 3).

Acceptance

• gate: fresh scaffold renders all zones empty-state with zero plugins
• gate: scaffold-static green