feat: Add multi-version Debian support (Trixie + Forky)

- Added support for Debian 14 (Forky) testing release
- Created examples for both Debian 13 (Trixie) and Debian 14 (Forky)
- Added comprehensive multi-version documentation
- Created automated version switching script with backup/restore
- Updated README with version-specific configuration examples
- Added migration guidance between Debian versions

This enables users to build images for both stable (Trixie) and
testing (Forky) releases with easy switching between versions.

examples/debian-basic.json

@@ -10,7 +10,10 @@
           "options": {
             "suite": "trixie",
             "mirror": "https://deb.debian.org/debian",
-            "components": ["main", "contrib", "non-free"]
+            "components": ["main", "contrib", "non-free"],
+            "additional_sources": [
+              "deb https://deb.debian.org/debian-security trixie-security main contrib non-free"
+            ]
           }
         },
         {
@@ -35,37 +38,45 @@
               "vim",
               "less",
               "locales",
-              "ca-certificates"
+              "ca-certificates",
+              "tzdata",
+              "net-tools",
+              "iproute2"
             ],
             "update": true,
             "clean": true
           }
         },
         {
-          "name": "org.osbuild.users",
+          "name": "org.osbuild.debian.locale",
+          "options": {
+            "language": "en_US.UTF-8",
+            "additional_locales": ["en_GB.UTF-8", "de_DE.UTF-8"],
+            "default_locale": "en_US.UTF-8"
+          }
+        },
+        {
+          "name": "org.osbuild.debian.timezone",
+          "options": {
+            "timezone": "UTC"
+          }
+        },
+        {
+          "name": "org.osbuild.debian.users",
           "options": {
             "users": {
               "debian": {
                 "password": "$6$rounds=656000$salt$hashedpassword",
                 "shell": "/bin/bash",
-                "groups": ["sudo", "users"],
+                "groups": ["sudo", "users", "adm"],
                 "uid": 1000,
                 "gid": 1000,
-                "home": "/home/debian"
+                "home": "/home/debian",
+                "comment": "Debian User"
               }
-            }
-          }
-        },
-        {
-          "name": "org.osbuild.locale",
-          "options": {
-            "language": "en_US.UTF-8"
-          }
-        },
-        {
-          "name": "org.osbuild.timezone",
-          "options": {
-            "timezone": "UTC"
+            },
+            "default_shell": "/bin/bash",
+            "default_home": "/home"
           }
         }
       ]
@@ -74,7 +85,6 @@
   "assembler": {
     "name": "org.osbuild.tar",
     "options": {
-      "filename": "debian-basic.tar.gz",
       "compression": "gzip"
     }
   }