fix(global_defaults): apply mirror default in pre-computed system_cfg path

roles/global_defaults/tasks/system.yml

@@ -29,6 +29,25 @@
   ansible.builtin.set_fact:
     system_cfg: "{{ system_defaults | combine(system | default({}), recursive=True) | combine(system_cfg, recursive=True) }}"
 
+- name: Apply mirror default for pre-computed system_cfg
+  when:
+    - system_cfg is defined
+    - _bootstrap_needs_enrichment | default(false) | bool
+    - system_cfg.mirror | default('') | string | trim | length == 0
+  vars:
+    # Same as _normalize_system.yml — kept in sync manually.
+    _mirror_defaults:
+      debian: "https://deb.debian.org/debian/"
+      ubuntu: "http://mirror.ubuntu.com/ubuntu/"
+      ubuntu-lts: "http://mirror.ubuntu.com/ubuntu/"
+  ansible.builtin.set_fact:
+    system_cfg: >-
+      {{
+        system_cfg | combine({
+          'mirror': _mirror_defaults[system_cfg.os | default('') | string | lower] | default('')
+        }, recursive=True)
+      }}
+
 - name: Derive convenience facts from pre-computed system_cfg
   when:
     - system_cfg is defined