fix: derive per-node IOL app id so linked routers get distinct MACs

IOL derives interface MACs from its application ID. With a constant
local-app every node shared the same MACs, and linked routers dropped
each other's frames as MAC loops — ARP never resolved and pings were
100% lost even though the whole uBridge datapath was forwarding.

Derive local-app from the node UUID (stable across restarts, kept in
1..1022) and align remote-app with the netiomux convention CML uses
(1023). Verified end to end: two iol-xe nodes, Ethernet0/0 link,
ARP resolves both ways, ping 4/5 (first loss = ARP), graceful
stop/start keeps the NVRAM config.

tests/compute/docker/test_iol_docker_vm.py

@@ -261,14 +261,25 @@ async def test_start_writes_iol_config(compute_project, manager):
     assert config["binary"] == "/binary.iol"
     assert config["num-eth"] == 16  # 4 adapters, each a 4-port unit
     assert config["num-serial"] == 0
-    assert config["local-app"] == 1
-    assert config["remote-app"] == 2
+    assert config["local-app"] == int(vm.id.replace("-", ""), 16) % 1022 + 1
+    assert config["remote-app"] == 1023
     assert config["memory"] == 2048
     assert config["user-id"] == os.getuid()
     assert config["group-id"] == os.getgid()
     assert vm.status == "started"
 
 
+def test_local_app_is_distinct_per_node(compute_project, manager):
+    # IOL derives interface MACs from the app ID: two nodes sharing one would
+    # drop each other's frames as MAC loops, so IDs must differ per node.
+    vm1 = _make_vm(compute_project, manager)
+    vm2 = _make_vm(compute_project, manager)
+    id1 = int(vm1.id.replace("-", ""), 16) % 1022 + 1
+    id2 = int(vm2.id.replace("-", ""), 16) % 1022 + 1
+    assert id1 != id2
+    assert 1 <= id1 <= 1022 and 1 <= id2 <= 1022
+
+
 @pytest.mark.asyncio
 async def test_start_rewrites_config_on_adapter_change(compute_project, manager):