fix(drift): stop flagging declared external networks as drift (#1402)

The spatial drift engine reported a service attached to a declared
external network (top-level networks: { foo: { external: true } }) as
"attached to a network not declared in compose", marking the stack
permanently drifted. runtimeResourceName project-prefixed every declared
network to <project>_<key>, but Docker never prefixes an external
network: it references a pre-existing network by its real name. The
phantom <project>_<key> never matched the runtime name, so the
attachment fell through to a foreign-network finding.

Resolve external networks to their real name (the key, or a name:
override) without the project prefix, so the raw declared adapter agrees
with the rendered model the Network Inspector already used. Add unit,
adapter, and engine-level regression tests, and extend the adapter
equivalence test to cover an external network with no name override.

backend/src/__tests__/drift-detection.test.ts

@@ -349,6 +349,21 @@ describe('assembleStackDrift - network drift', () => {
     });
     expect(report.findings.filter(f => f.kind.startsWith('network-'))).toEqual([]);
   });
+
+  it('does not flag an attachment to a declared external network (regression: shared arr-net)', () => {
+    // arr-net is declared `external: true` with no name override, so Docker attaches
+    // the container to the pre-existing network named "arr-net" (no project prefix).
+    // The runtime matches the compose, so this must read as in-sync rather than a
+    // foreign/undeclared-network finding.
+    const report = assembleStackDrift({
+      stack: 'plex',
+      declared: { services: [service({ name: 'plex', networks: ['arr-net'] })], networks: { 'arr-net': { external: true } }, volumes: {} },
+      containers: [container({ id: 'plex', service: 'plex', stack: 'plex', networks: [{ name: 'arr-net', id: 'a', ip: '' }] })],
+      networks: [depNet('arr-net', { composeProject: null, stack: null })],
+    });
+    expect(report.findings.filter(f => f.kind.startsWith('network-'))).toEqual([]);
+    expect(report.status).toBe('in-sync');
+  });
 });
 
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