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.Configuration/Validation/ValidationError.cs

@@ -0,0 +1,8 @@
+namespace ZB.MOM.WW.OtOpcUa.Configuration.Validation;
+
+/// <summary>
+///     One validation failure. <see cref="Code"/> is a stable machine-readable symbol
+///     (<c>BadCrossClusterNamespaceBinding</c>, <c>UnsSegmentInvalid</c>, …). <see cref="Context"/>
+///     carries the offending logical ID so the Admin UI can link straight to the row.
+/// </summary>
+public sealed record ValidationError(string Code, string Message, string? Context = null);