refactor(data): address occurrences by (taskId, occurrenceStart)

Phase 0 of docs/OWN-STORE.md. Prepares the seam for the Room store while
the provider is still the store.

Task.id (the materialised instance row id) is gone; Task carries
occurrenceStart, its RECURRENCE-ID anchor, instead. updateInstance takes
(taskId, occurrenceStart, form) and AndroidTasksDataSource maps that back
to an instance row itself, so the provider path exercises the new
signature before Room exists.

Lazy-list keys move to Task.occurrenceKey. Two occurrences of one series
can appear in the same list once expansion is ours, and taskId alone
would collide there.

domain/Models.kt stops importing TasksContract — status, priority and
local-account constants now live in domain. StorageMode gains OWN as a
third value; LOCAL keeps meaning the dmfs provider until it is deleted.

Room 2.8.4 and room.schemaLocation added to the build.

app/build.gradle.kts

@@ -137,6 +137,13 @@ kotlin {
     }
 }
 
+// Export each Room schema version to app/schemas/ and commit it. That JSON is
+// what MigrationTestHelper reads to build an old database and migrate it, so
+// without it a migration can only be tested by hand.
+ksp {
+    arg("room.schemaLocation", "$projectDir/schemas")
+}
+
 dependencies {
     // Agendula's own task store — the dmfs provider vendored under our authority.
     // Contributes a <provider> to the merged manifest; no app code imports from it
@@ -162,6 +169,10 @@ dependencies {
     implementation(libs.androidx.navigation.compose)
     ksp(libs.hilt.compiler)
 
+    implementation(libs.androidx.room.runtime)
+    implementation(libs.androidx.room.ktx)
+    ksp(libs.androidx.room.compiler)
+
     implementation(libs.androidx.datastore.preferences)
     implementation(libs.androidx.documentfile)