keyboard: fix interactive dismiss race conditions, rewrite example

Fix several race conditions in the iOS interactive dismiss flow:
- Don't skip keyboard close notifications during pan tracking, which
  left Dart unaware the keyboard was dismissed
- Guard resignFirstResponder with generation check so reopened keyboards
  aren't killed by stale dismiss completions
- Block pan tracking from starting during an in-flight dismiss animation
- Always reset keyboard view bounds when the keyboard opens, not just
  when the dismiss animator is still running
- Handle duration=0 keyboard notifications by snapping immediately
- Gate adaptive learning debug output behind kDebugMode

Rewrite the example as a chat UI demonstrating ListenableBuilder,
scroll freeze during interactive dismiss, and proper bottom inset
handling with max(keyboardHeight, safeBottom).

Modernize example Android project from v1 to v2 embedding with
current AGP/Gradle versions.

example/.metadata

@@ -4,7 +4,27 @@
 # This file should be version controlled and should not be manually edited.
 
 version:
-  revision: 20e59316b8b8474554b38493b8ca888794b0234a
-  channel: v1.7.8-hotfixes
+  revision: "2c9eb20739dfec95e2c74bd3dfa4601b0a8a36aa"
+  channel: "[user-branch]"
 
 project_type: app
+
+# Tracks metadata for the flutter migrate command
+migration:
+  platforms:
+    - platform: root
+      create_revision: 2c9eb20739dfec95e2c74bd3dfa4601b0a8a36aa
+      base_revision: 2c9eb20739dfec95e2c74bd3dfa4601b0a8a36aa
+    - platform: macos
+      create_revision: 2c9eb20739dfec95e2c74bd3dfa4601b0a8a36aa
+      base_revision: 2c9eb20739dfec95e2c74bd3dfa4601b0a8a36aa
+
+  # User provided section
+
+  # List of Local paths (relative to this file) that should be
+  # ignored by the migrate tool.
+  #
+  # Files that are not part of the templates will be ignored by default.
+  unmanaged_files:
+    - 'lib/main.dart'
+    - 'ios/Runner.xcodeproj/project.pbxproj'