feat(ios): give Room a way to construct the database without reflection

`MantraDatabaseConstructor` has been in commonMain all along -- `expect`,
`@Suppress`ed for having no actual, and referenced by nothing. Room's KSP
only generates the actual when `@ConstructedBy` names the object, so on a
target with no reflection there was no way into the database at all.

This is the one annotation in the ios work that changes every other
target too: the generator now emits an `actual object` for android and
jvm as well, each returning `MantraDatabase_Impl()`, where before it
emitted none. jvmTest is unmoved at 1174, and MantraDatabaseJvmTest's
comment about the no-Context builder overload being available "because
MantraDatabaseConstructor gives Room a way to instantiate without
reflection" is now true rather than aspirational.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

composeApp/src/commonMain/kotlin/press/mantra/compose/database/MantraDatabase.kt

@@ -2,6 +2,7 @@ package press.mantra.compose.database
 
 import androidx.room3.ColumnTypeConverters
 import androidx.room3.AutoMigration
+import androidx.room3.ConstructedBy
 import androidx.room3.Database
 import androidx.room3.RoomDatabase
 import androidx.room3.immediateTransaction
@@ -366,6 +367,7 @@ val GENESIS_AT = Instant.fromEpochMilliseconds(1231006505000L)
         AutoMigration(from = 21, to = 22),
     ]
 )
+@ConstructedBy(MantraDatabaseConstructor::class)
 @ColumnTypeConverters(MantraConverters::class)
 abstract class MantraDatabase: RoomDatabase() {
     abstract fun broadcastNostrEventReceiptDao(): BroadcastNostrEventReceiptDao