refactor(global_defaults): extract OS family lists to single source of truth

roles/global_defaults/defaults/main.yml

@@ -1,4 +1,27 @@
 ---
+# OS family lists — single source of truth for platform detection and validation
+os_family_rhel:
+  - almalinux
+  - fedora
+  - rhel
+  - rocky
+os_family_debian:
+  - debian
+  - ubuntu
+  - ubuntu-lts
+os_supported:
+  - almalinux
+  - alpine
+  - archlinux
+  - debian
+  - fedora
+  - opensuse
+  - rhel
+  - rocky
+  - ubuntu
+  - ubuntu-lts
+  - void
+
 # User input. Normalized into hypervisor_cfg + hypervisor_type.
 hypervisor:
   type: "none"