[frontend-contrib S8] Frontend registry emissions: transactional replace-set

[rickylabs]

Part of #922

Scope

generate plugins emits the full frontend set (registry/islands/routes/css/gateway/check) staged -> deno check (frontend.check.ts imports every referenced module) -> atomic swap; deterministic empty emissions; regeneration on plugin install/update/remove; byte-identical skip; full collision rules (route patterns, reserved paths, basePath).

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

Acceptance

• gate: type-broken contribution fails install with real diagnostic
• gate: plugin remove leaves zero dangling imports (empty-set emission test)
• gate: staged failure leaves previous set untouched