feat(server-02): promote EventStore to global-sequence store with subscribeAll

StoredEvent gains sessionSequence: Long (per-session monotonic); existing
sequence field becomes the global monotonic cursor across the entire store.
SqliteEventStore allocates both counters inside the same BEGIN IMMEDIATE
transaction and emits to a global MutableSharedFlow after commit.
InMemoryEventStore mirrors this with an AtomicLong global counter and a
suspend-on-overflow SharedFlow. EventStore interface gains subscribeAll()
and lastGlobalSequence(). Contract tests updated and extended to cover
global-sequence and cross-session subscribeAll invariants.

core/events/src/main/kotlin/com/correx/core/events/events/EventEnvelope.kt

@@ -12,5 +12,6 @@ data class NewEvent(
 data class StoredEvent(
     val metadata: EventMetadata,
     val sequence: Long,
+    val sessionSequence: Long,
     val payload: EventPayload
 )

core/events/src/main/kotlin/com/correx/core/events/stores/EventStore.kt

@@ -42,6 +42,10 @@ interface EventStore {
      */
     fun subscribe(sessionId: SessionId): Flow<StoredEvent>
 
+    fun subscribeAll(): Flow<StoredEvent>
+
+    suspend fun lastGlobalSequence(): Long
+
     /**
      * Iterate every event in the store across all sessions, in monotonic insertion order.
      * Intended for batch jobs (e.g. CAS liveness scans). Implementations should stream lazily.