chore: organize solution into module folders (Core/Server/Drivers/Client/Tooling)

Group all 69 projects into category subfolders under src/ and tests/ so the
Rider Solution Explorer mirrors the module structure. Folders: Core, Server,
Drivers (with a nested Driver CLIs subfolder), Client, Tooling.

- Move every project folder on disk with git mv (history preserved as renames).
- Recompute relative paths in 57 .csproj files: cross-category ProjectReferences,
  the lib/ HintPath+None refs in Driver.Historian.Wonderware, and the external
  mxaccessgw refs in Driver.Galaxy and its test project.
- Rebuild ZB.MOM.WW.OtOpcUa.slnx with nested solution folders.
- Re-prefix project paths in functional scripts (e2e, compliance, smoke SQL,
  integration, install).

Build green (0 errors); unit tests pass. Docs left for a separate pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/Core/ZB.MOM.WW.OtOpcUa.Core.Tests/DriverHostTests.cs

@@ -0,0 +1,80 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Core.Hosting;
+
+namespace ZB.MOM.WW.OtOpcUa.Core.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class DriverHostTests
+{
+    private sealed class StubDriver(string id, bool failInit = false) : IDriver
+    {
+        public string DriverInstanceId { get; } = id;
+        public string DriverType => "Stub";
+        public bool Initialized { get; private set; }
+        public bool ShutDown { get; private set; }
+
+        public Task InitializeAsync(string _, CancellationToken ct)
+        {
+            if (failInit) throw new InvalidOperationException("boom");
+            Initialized = true;
+            return Task.CompletedTask;
+        }
+
+        public Task ReinitializeAsync(string _, CancellationToken ct) => Task.CompletedTask;
+        public Task ShutdownAsync(CancellationToken ct) { ShutDown = true; return Task.CompletedTask; }
+        public DriverHealth GetHealth() =>
+            new(Initialized ? DriverState.Healthy : DriverState.Unknown, null, null);
+        public long GetMemoryFootprint() => 0;
+        public Task FlushOptionalCachesAsync(CancellationToken ct) => Task.CompletedTask;
+    }
+
+    [Fact]
+    public async Task Register_initializes_driver_and_tracks_health()
+    {
+        await using var host = new DriverHost();
+        var driver = new StubDriver("d-1");
+
+        await host.RegisterAsync(driver, "{}", CancellationToken.None);
+
+        host.RegisteredDriverIds.ShouldContain("d-1");
+        driver.Initialized.ShouldBeTrue();
+        host.GetHealth("d-1")!.State.ShouldBe(DriverState.Healthy);
+    }
+
+    [Fact]
+    public async Task Register_rethrows_init_failure_but_keeps_driver_registered()
+    {
+        await using var host = new DriverHost();
+        var driver = new StubDriver("d-bad", failInit: true);
+
+        await Should.ThrowAsync<InvalidOperationException>(() =>
+            host.RegisterAsync(driver, "{}", CancellationToken.None));
+
+        host.RegisteredDriverIds.ShouldContain("d-bad");
+    }
+
+    [Fact]
+    public async Task Duplicate_registration_throws()
+    {
+        await using var host = new DriverHost();
+        await host.RegisterAsync(new StubDriver("d-1"), "{}", CancellationToken.None);
+
+        await Should.ThrowAsync<InvalidOperationException>(() =>
+            host.RegisterAsync(new StubDriver("d-1"), "{}", CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task Unregister_shuts_down_and_removes()
+    {
+        await using var host = new DriverHost();
+        var driver = new StubDriver("d-1");
+        await host.RegisterAsync(driver, "{}", CancellationToken.None);
+
+        await host.UnregisterAsync("d-1", CancellationToken.None);
+
+        host.RegisteredDriverIds.ShouldNotContain("d-1");
+        driver.ShutDown.ShouldBeTrue();
+    }
+}