feat: add model download controller

Refs #135

Author: leehack

CHANGELOG.md

@@ -4,6 +4,14 @@
   * Added WebGPU readiness guidance covering browser capability checks,
     cross-origin isolation, bridge asset/version diagnostics, fallback behavior,
     model/configuration pressure, and the Flutter Web real-model smoke path.
+* **Model download UX**:
+  * Added `ModelDownloadController`, a dependency-free helper that turns
+    `ModelDownloadManager` cache/download work into app-facing lifecycle states
+    for resolving, cache checks, downloads, verification, ready, failed,
+    cancelled, and retry flows.
+  * Wired the runnable chat app example through a `ModelDownloadManager` adapter
+    so its model-management UI demonstrates the controller while preserving the
+    example's multi-asset and web-cache service behavior.
 
 ## 0.6.13