[frontend-contrib S6] @netscript/plugin-frontend-core contracts/v1

[rickylabs]

Part of #922

Scope

New Archetype-1 package: FrontendManifestEnvelope + (family, major) versioning, app-family kinds (route/island/zone/nav/theme), identity quartet, HostSurfaceDescriptor, PluginRequestContext/PluginClientContext split, FrontendRequires, FrontendBudgets, defineFrontend (FrontendDefinition authoring form: contract default, string MessageRef shorthand, singular theme).

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

Acceptance

• gate: deno doc --lint clean; jsr publish dry-run green
• gate: envelope negotiation tests (old-host/new-plugin x new-host/old-plugin)
• gate: quality:scan + arch:check green