refactor(quartz): intern only on store reads, inject per-store interner

The previous commit interned at Event.fromJson, which is too early —
the deserializer can produce events with manipulated id fields that
don't match their content. Move interning to the post-validation
boundary: events are only canonicalised when read back from durable
storage, where they were valid when written.

- Revert Event.fromJson to plain OptimizedJsonMapper.fromJson.
- Revert ObservableEventStore.insert interning. Substituting the
  caller's event for a previously-cached one with the same id but
  potentially different sig was a write-side surprise; we leave the
  caller's instance alone.
- Intern at SQLiteStatement.toEvent (read deserialization).
- Intern at FsEventStore.readEvent (FS reads).

The interner is also now constructor-injected through every store
layer (SQLiteEventStore, EventStore, QueryBuilder, FsEventStore)
defaulting to EventInterner.Default. BaseDBTest gives each parallel
forEachDB store its own EventInterner — without this, parallel test
runs that sign "same id, different sig" events through the shared
Default would see the first run's sig leak across all DBs.

All 240 store + projection + interner tests pass.

https://claude.ai/code/session_01Jny85MTu1ynKgFBgysfWu5

quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip01Core/store/sqlite/BaseDBTest.kt

@@ -20,6 +20,7 @@
  */
 package com.vitorpamplona.quartz.nip01Core.store.sqlite
 
+import com.vitorpamplona.quartz.nip01Core.core.EventInterner
 import com.vitorpamplona.quartz.utils.Secp256k1Instance
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.launch
@@ -63,6 +64,12 @@ open class BaseDBTest {
                             EventStore(
                                 dbName = null,
                                 indexStrategy = indexStrategy,
+                                // Each store gets its own interner so
+                                // parallel forEachDB runs don't share a
+                                // canonical Event for the same id —
+                                // tests sign with random sigs that
+                                // would otherwise cross-pollinate.
+                                interner = EventInterner(),
                             )
                     }
                 }