feat(secrets): G-2a secret: arm on GalaxySecretRef via ISecretResolver (Task 7)

Add a secret:NAME arm to GalaxySecretRef.ResolveApiKey that resolves the Galaxy
gateway API key through the shared ISecretResolver — fail-closed if the secret is
absent (never falls through to the cleartext literal arm), retiring the dev:/literal
in-DB path for production. Because GetAsync is async the method becomes
ResolveApiKeyAsync; the await cascade threads ISecretResolver by ctor injection into
GalaxyDriver + GalaxyDriverBrowser and (since GalaxyDriver is built by a static
factory closure, not DI) through GalaxyDriverFactoryExtensions + DriverFactoryBootstrap
(which pulls the real resolver from the service provider — registered unconditionally
in Slice 1). A NullSecretResolver null-object backs the parse-only/test paths only;
the runtime path always gets the real resolver (verified end-to-end).

TDD: 3 new secret:-arm tests (resolve / fail-closed-on-absent / no-literal-warning)
RED without the arm, GREEN with it; 338 Galaxy tests pass; no sync-over-async.

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GalaxyDriverReadTests.cs

@@ -78,7 +78,7 @@ public sealed class GalaxyDriverReadTests
         // _dataReader and _subscriber are both null. The follow-up read path can't
         // synthesise a Read without one, so it surfaces a NotSupportedException
         // pointing at the misuse rather than NullRef'ing inside the pump path.
-        var driver = new GalaxyDriver("g", Opts());
+        var driver = new GalaxyDriver("g", Opts(), hierarchySource: null);
 
         var ex = await Should.ThrowAsync<NotSupportedException>(() =>
             driver.ReadAsync(["x"], CancellationToken.None));