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/fixtures/src/main/kotlin/com/correx/testing/fixtures/EventFixtures.kt

@@ -34,6 +34,7 @@ object EventFixtures {
         payload: EventPayload = ToolInvokedEvent("write"),
         sessionId: SessionId = SessionId("s1"),
         sequence: Long = 1L,
+        sessionSequence: Long = sequence,
         timestamp: Instant = Instant.parse("2026-01-01T00:00:00Z")
     ): StoredEvent {
         return StoredEvent(
@@ -46,8 +47,8 @@ object EventFixtures {
                 correlationId = null
             ),
             sequence = sequence,
+            sessionSequence = sessionSequence,
             payload = payload
         )
-
     }
 }