M1: full task data layer, reminders and ViewModels over OpenTasks

Non-visual stack (backoffice) complete and verified against the real
provider semantics (tasks.org's bundled dmfs TaskProvider, authority
org.tasks.opentasks, org.tasks.permission.* dangerous):

- TaskContract subset + ProviderResolver (runtime authority/permission)
- AndroidTasksDataSource (Instances query, ContentObserver) + TasksRepository
  with live Flows; domain models, smart-list filtering, sorting, form validation
- Self-scheduled due-reminder engine (AlarmManager, boot/provider-change)
- Render-only ViewModels + UiState for every screen
- 24 JVM unit tests; assembleDebug + lintDebug + testDebugUnitTest green
- RootScreen is a functional scaffold over real data, to be replaced with the
  Material 3 Expressive screens one by one

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/res/drawable/ic_notification.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Notification status-bar icon: a simple check, tinted white by the system. -->
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="24"
+    android:viewportHeight="24"
+    android:tint="#FFFFFFFF">
+    <path
+        android:fillColor="#FF000000"
+        android:pathData="M9,16.17 L4.83,12 l-1.42,1.41 L9,19 21,7 l-1.41,-1.41z" />
+</vector>