fix(store): three defects the instrumented suite found on first run

- forking an occurrence copied the master's alarm row id and hit the
  primary key; replaceForTask now clears it
- a sub-second DTSTART made lib-recur emit the anchor and its truncated
  self, doubling a series' first occurrence; floor to the second, which
  is all RFC 5545 DATE-TIME carries
- room-testing needs kotlinx-serialization 1.8+, but consistent
  resolution pinned androidTest to the app's 1.7.3

Tests: 52 pass on device.

app/build.gradle.kts

@@ -149,6 +149,14 @@ ksp {
 }
 
 dependencies {
+    // Not a dependency we use directly — lifecycle already drags it in at 1.7.3.
+    // AGP's consistent resolution then pins androidTest to the app classpath, and
+    // room-testing's MigrationTestHelper needs 1.8+ to deserialize the exported
+    // schema; on 1.7.3 it dies with an AbstractMethodError. Raise it in one place.
+    constraints {
+        implementation(libs.kotlinx.serialization.json)
+    }
+
     implementation(libs.androidx.core.ktx)
     implementation(libs.androidx.appcompat)
     implementation(libs.androidx.lifecycle.runtime.ktx)