feat: gate_bus mask stash

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

gates/gate_bus.py

@@ -33,3 +33,11 @@ class GateBus:
                 raise GateCancelled()
             time.sleep(period)
         return cls.messages.pop(sid)
+
+    @classmethod
+    def put_mask(cls, node_id, data):
+        cls.masks[str(node_id)] = data
+
+    @classmethod
+    def pop_mask(cls, node_id):
+        return cls.masks.pop(str(node_id), None)

tests/test_gate_bus.py

@@ -26,3 +26,13 @@ def test_arm_clears_stale_state():
     gb.GateBus.arm("1")
     assert "1" not in gb.GateBus.messages
     assert gb.GateBus.cancelled is False
+
+def test_mask_stash_roundtrip():
+    gb.GateBus.put_mask("9", b"PNGDATA")
+    assert gb.GateBus.pop_mask("9") == b"PNGDATA"
+    assert gb.GateBus.pop_mask("9") is None   # popped
+
+def test_arm_clears_mask():
+    gb.GateBus.put_mask("9", b"x")
+    gb.GateBus.arm("9")
+    assert gb.GateBus.pop_mask("9") is None