feat(runtime): 정적 프런트엔드와 격리 빌더를 구성한다

Caddy가 불변 아티팩트를 직접 제공하고 runtime과 builder의 자원·비밀 경계를 분리한다.

Gateway active release ref를 재기동에 복원하고 production/development/smoke 모델 검증을 확장한다.

compose.dev.yaml

@@ -3,12 +3,20 @@ services:
     restart: "no"
   redis:
     restart: "no"
+  builder:
+    user: ${DEV_UID:-1000}:${DEV_GID:-1000}
+    volumes:
+      - ${CORE_DEV_PATH:-../core2026}:/workspace/core2026
+      - core-source:/srv/core
+      - builder-cache:/srv/builder-cache
+    restart: "no"
   runtime:
     user: ${DEV_UID:-1000}:${DEV_GID:-1000}
     environment:
       CORE_SOURCE_MODE: bind
       CORE_BIND_ROOT: /workspace/core2026
       RUNTIME_MODE: development
+      FRONTEND_SERVE_MODE: preview
       VITE_PREVIEW_ALLOWED_HOSTS: '*'
     volumes:
       - ${CORE_DEV_PATH:-../core2026}:/workspace/core2026