[frontend-contrib S19] generate frontend-wiring adoption verb

[rickylabs]

Part of #922

Scope

Idempotent adoption for EXISTING apps: vite islandSpecifiers feed, router plugin-routes merge, PluginZone hints, css import — the non-breaking migration path the RFC promises.

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

Acceptance

• gate: pre-layer scaffolded app adopts and passes the extended scaffold.runtime assertions
• gate: second run is a byte-identical no-op