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.

testing/deterministic/src/test/kotlin/RouterFacadeTest.kt

@@ -606,7 +606,8 @@ class RouterFacadeTest {
             appendedEvents.add(event)
             val stored = StoredEvent(
                 metadata = event.metadata,
-                sequence = nextSequence++,
+                sequence = nextSequence,
+                sessionSequence = nextSequence++,
                 payload = event.payload,
             )
             storedEvents[event.metadata.eventId] = stored
@@ -635,5 +636,9 @@ class RouterFacadeTest {
             storedEvents.values.asSequence()
 
         override fun allSessionIds(): Set<SessionId> = storedEvents.values.map { it.metadata.sessionId }.toSet()
+
+        override fun subscribeAll(): Flow<StoredEvent> = TODO("Not needed in this test context")
+
+        override suspend fun lastGlobalSequence(): Long = TODO("Not needed in this test context")
     }
 }