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.

gradle/libs.versions.toml

@@ -15,6 +15,7 @@ material3 = "1.5.0-alpha21"
 datastore = "1.2.1"
 # Room — Agendula's own task store (docs/OWN-STORE.md).
 room = "2.8.4"
+kotlinxSerialization = "1.8.1"
 # SAF directory writing for export/backup (DocumentFile).
 documentfile = "1.1.0"
 junit = "6.1.0"
@@ -78,6 +79,7 @@ androidx-room-runtime = { group = "androidx.room", name = "room-runtime", versio
 androidx-room-ktx = { group = "androidx.room", name = "room-ktx", version.ref = "room" }
 androidx-room-compiler = { group = "androidx.room", name = "room-compiler", version.ref = "room" }
 androidx-room-testing = { group = "androidx.room", name = "room-testing", version.ref = "room" }
+kotlinx-serialization-json = { group = "org.jetbrains.kotlinx", name = "kotlinx-serialization-json", version.ref = "kotlinxSerialization" }
 
 # DataStore
 androidx-datastore-preferences = { group = "androidx.datastore", name = "datastore-preferences", version.ref = "datastore" }