refactor: extract BLE protocol stack into meshcore_client package

Move BLE communication layer (command queue, frame parser/builder,
protocol constants, data models) into a standalone reusable Dart package
at ../meshcore_client. App model files become thin re-export wrappers,
keeping all existing consumers working without import changes.

pubspec.lock

@@ -589,6 +589,13 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.4.2"
+  meshcore_client:
+    dependency: "direct main"
+    description:
+      path: "../meshcore_client"
+      relative: true
+    source: path
+    version: "0.1.0"
   meta:
     dependency: transitive
     description: