docs(bootstrap): add section comments, role boundary docs, and pipeline overview

roles/global_defaults/tasks/system.yml

@@ -1,4 +1,10 @@
 ---
+# Two code paths:
+#   1. Fresh run (system_cfg undefined): normalize from raw `system` input.
+#   2. Pre-computed (system_cfg already set, e.g. from main project's deploy_iac):
+#      merge with bootstrap system_defaults to fill missing fields (luks, features,
+#      etc.) that bootstrap expects but the main project doesn't set, then derive
+#      convenience facts (hostname, os, os_version).
 - name: Normalize system and disk configuration
   when: system_cfg is not defined
   block: