feat: add Ubuntu 26.04 (Resolute) support

Add Dockerfile, package script, and CI/build configuration for ubuntu-2604 across all build targets.

test/docker-compose.yml

@@ -23,6 +23,14 @@ services:
       - R_VERSION=${R_VERSION}
     volumes:
       - ../:/r-builds
+  ubuntu-2604:
+    image: ubuntu:resolute
+    command: /r-builds/test/test-apt.sh
+    environment:
+      - OS_IDENTIFIER=ubuntu-2604
+      - R_VERSION=${R_VERSION}
+    volumes:
+      - ../:/r-builds
   debian-12:
     image: debian:bookworm
     command: /r-builds/test/test-apt.sh