[frontend-contrib S1] P1 proof: mounted sub-app command ordering

[rickylabs]

Part of #922

Scope

Disposable proof for Fresh 2.3.3 mount semantics: host _app/nested layouts/basePath × plugin middleware-before-routes × plugin 404 behavior × two plugins mounted via App.mountApp in a post-fsRoutes composition phase; child App-level commands stripped. Decides D6; fallback = prefixed app.route() without mountApp.

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

Acceptance

• gate: integration fixtures cover host _app wrap, nested layouts, basePath, plugin middleware, plugin 404, two plugins
• gate: pass/fail verdict + chosen mount mechanism recorded in the run dir and epic