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>

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/ISubscribable.cs

@@ -0,0 +1,47 @@
+namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+/// <summary>
+///     Driver capability for data-change subscriptions — covers both native subscriptions
+///     (Galaxy MXAccess advisory, OPC UA monitored items, TwinCAT ADS notifications) and
+///     driver-internal polled subscriptions (Modbus, AB CIP, S7, FOCAS). The driver owns
+///     its polling loop where applicable; the Core just sees <see cref="OnDataChange"/>
+///     callbacks regardless of mechanism.
+/// </summary>
+public interface ISubscribable
+{
+    /// <summary>
+    ///     Subscribe to data changes for a batch of attributes.
+    ///     The driver MAY fire <see cref="OnDataChange"/> immediately with the current value
+    ///     (initial-data callback per OPC UA convention) and again on every change.
+    /// </summary>
+    /// <returns>An opaque subscription handle the caller passes to <see cref="UnsubscribeAsync"/>.</returns>
+    Task<ISubscriptionHandle> SubscribeAsync(
+        IReadOnlyList<string> fullReferences,
+        TimeSpan publishingInterval,
+        CancellationToken cancellationToken);
+
+    /// <summary>Cancel a subscription returned by <see cref="SubscribeAsync"/>.</summary>
+    Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken);
+
+    /// <summary>
+    ///     Server-pushed data-change notification. Fires whenever a subscribed attribute changes,
+    ///     and (per OPC UA convention) on subscription establishment for current values.
+    /// </summary>
+    event EventHandler<DataChangeEventArgs>? OnDataChange;
+}
+
+/// <summary>Opaque subscription identity returned by <see cref="ISubscribable.SubscribeAsync"/>.</summary>
+public interface ISubscriptionHandle
+{
+    /// <summary>Driver-internal subscription identifier (for diagnostics + post-mortem).</summary>
+    string DiagnosticId { get; }
+}
+
+/// <summary>Event payload for <see cref="ISubscribable.OnDataChange"/>.</summary>
+/// <param name="SubscriptionHandle">The handle returned by the original <see cref="ISubscribable.SubscribeAsync"/> call.</param>
+/// <param name="FullReference">Driver-side full reference of the changed attribute.</param>
+/// <param name="Snapshot">New value + quality + timestamps.</param>
+public sealed record DataChangeEventArgs(
+    ISubscriptionHandle SubscriptionHandle,
+    string FullReference,
+    DataValueSnapshot Snapshot);