refactor(global_defaults): add idempotency guards to normalization tasks

roles/global_defaults/tasks/hypervisor.yml

@@ -1,4 +1,7 @@
 ---
+- name: Normalize hypervisor configuration
+  when: hypervisor_cfg is not defined
+  block:
     - name: Ensure hypervisor input is a dictionary
       ansible.builtin.set_fact:
         hypervisor: "{{ hypervisor | default({}) }}"
@@ -12,7 +15,7 @@
         fail_msg: "hypervisor must be a dictionary and hypervisor.type must be set (e.g. libvirt|proxmox|vmware|xen|none)."
         quiet: true
 
-- name: Normalize hypervisor configuration
+    - name: Merge hypervisor defaults with input
       vars:
         merged: "{{ hypervisor_defaults | combine(hypervisor, recursive=True) }}"
       ansible.builtin.set_fact:

roles/global_defaults/tasks/system.yml

@@ -1,4 +1,7 @@
 ---
+- name: Normalize system and disk configuration
+  when: system_cfg is not defined
+  block:
     - name: Validate raw system input types
       ansible.builtin.include_tasks: _validate_input.yml