feat(deploy,runtime): wire mxaccessgw connection — endpoint, key, seed row

User confirmed the mxaccessgw client (Galaxy driver) doesn't need Windows
— only the gateway worker has that constraint. This wires the Galaxy
driver into the docker-dev fleet:

- docker-compose.yml: GALAXY_MXGW_API_KEY env var on every host service
  (admin nodes harmlessly ignore it; driver-role nodes pick it up when
  the seeded DriverInstance resolves ApiKeySecretRef=env:GALAXY_MXGW_API_KEY).
  Default value matches the key the operator provided; override via shell
  env (GALAXY_MXGW_API_KEY=... docker compose up -d) to rotate without
  editing compose.
- seed-clusters.sql: now creates a SystemPlatform Namespace
  (MAIN-galaxy, urn:zb:docker-dev:galaxy) plus a GalaxyMxGateway
  DriverInstance (MAIN-galaxy-mxgw) in the MAIN cluster pointing at
  http://10.100.0.48:5120 with UseTls=false. Idempotent via IF NOT EXISTS.
- DriverInstanceActor.ShouldStub: clarified the doc comment — only the
  legacy "Galaxy" type name and "Historian.Wonderware" are Windows-only;
  the v2 "GalaxyMxGateway" driver is .NET 10 cross-platform (gRPC to an
  external gateway) and is NOT stubbed.
- README: documents the final operator step — sign in, click "Deploy
  current configuration" on /deployments to materialise the seeded
  Galaxy driver into a running gRPC connection. Raw DriverInstance rows
  don't spawn drivers on their own; the v2 lifecycle requires a sealed
  Deployment first.

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverInstanceActor.cs

@@ -66,9 +66,15 @@ public sealed class DriverInstanceActor : ReceiveActor, IWithTimers
 
     /// <summary>
     /// Returns true when the driver should boot in DEV-STUB mode based on host platform and
-    /// configured roles. Mirrors plan §Task 55: Windows-only driver types (Galaxy, Wonderware
-    /// Historian) are stubbed when running on non-Windows OR when the host carries the
-    /// <c>dev</c> role.
+    /// configured roles. Only the v1 in-process types stay Windows-only:
+    /// <list type="bullet">
+    ///   <item><c>"Galaxy"</c> — legacy MXAccess COM proxy (retired in PR 7.2; gated for any
+    ///         leftover DriverInstance rows that still reference the old type name).</item>
+    ///   <item><c>"Historian.Wonderware"</c> — Wonderware Historian sidecar over Windows-only
+    ///         named pipes.</item>
+    /// </list>
+    /// The v2 <c>"GalaxyMxGateway"</c> driver talks gRPC to an external mxaccessgw process,
+    /// so it runs on any platform .NET 10 supports — Linux containers included. Not stubbed.
     /// </summary>
     public static bool ShouldStub(string driverType, IEnumerable<string> roles)
     {