[frontend-contrib S9] @netscript/fresh/plugins host runtime

[rickylabs]

Part of #922

Scope

Post-fsRoutes composition phase on defineFreshApp({ frontend }), mountPluginFrontends (middleware-then-routes, App-command stripping), normalizeFreshRouteModule, PluginZone (deterministic order, provenance wrappers, quarantine card), pluginNavSections, definePluginPage (PluginPageContext: host/client/redirect), pluginApi(client), GATEWAY_PREFIX constant.

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

Acceptance

• gate: P1/P2/P4 proof fixtures re-run green against the real implementation
• gate: SCOPE-frontend browser gates (route renders, states, responsive)
• gate: quality:scan + arch:check green