Optimize storage handling and drop redundant runtime state

[usmanvrtx]

Description

This PR adds runtime support for model-specific page events, strengthens API and widget lifecycle handling, and storage improvements.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

What Has Changed

• Added PageEventBus to support model-specific event handling.
• Improved API disposal guards and widget rebinding behavior.
• Updated storage behavior and added focused tests for the new runtime behavior.

How to Test

1. Run flutter test test/page_event_bus_test.dart test/api_dispose_guard_test.dart test/ensemble_widget_rebind_test.dart test/storage_manager_test.dart from modules/ensemble.
2. Run the TV focus tests from modules/ensemble: flutter test test/framework/tv/page_tv_header_test.dart test/widget/tv_focus_registry_index_test.dart test/widget/tv_focus_registry_test.dart test/widget/tv_focus_scroll_test.dart.
3. Run the full flutter test suite from modules/ensemble.

Screenshots / Videos

Not applicable.

Checklist

• I have run flutter analyze and addressed any new warnings
• I have run flutter test and all tests pass
• I have tested my changes on the relevant platform(s)
• I have updated documentation if needed
• My changes do not introduce new warnings or errors