fix: preserve unmanaged first-seen timestamps

Co-authored-by: genichin <4282227+genichin@users.noreply.github.com>

tests/application/test_unmanaged_position_lifecycle.py

@@ -109,6 +109,22 @@ def test_in_memory_store_isolates_same_symbol_by_instance():
     assert state_b["quantity"] == 7
 
 
+def test_in_memory_store_preserves_first_seen_when_resolved_symbol_reappears():
+    store = InMemoryUnmanagedPositionStore()
+    t1 = datetime(2026, 9, 7, 0, 0, tzinfo=UTC)
+    t2 = t1 + timedelta(minutes=1)
+    t3 = t2 + timedelta(minutes=1)
+
+    store.observe(instance_key=INSTANCE, symbol="008110", quantity=3, observed_at=t1)
+    store.resolve_absent(instance_key=INSTANCE, observed_symbols=set(), observed_at=t2)
+    reopened = store.observe(instance_key=INSTANCE, symbol="008110", quantity=5, observed_at=t3)
+
+    assert reopened["transition"] == "detected"
+    assert reopened["state"]["first_seen_at"] == t1
+    assert reopened["state"]["last_seen_at"] == t3
+    assert reopened["state"]["quantity_changed_at"] == t3
+
+
 def test_reconcile_core_holdings_suppresses_repeated_unmanaged_alarm_spam():
     repo, svc = _wire()
     emitter, transport = _make_emitter()