Parallelize DC discovery and improve diagnostics and UI reliability

[zbalkan]

Summary

This PR delivers a major reliability and performance update for domain controller discovery and replication checks. It parallelizes long-running discovery operations, improves progress reporting, routes DC-override discovery through the selected domain controller, and strengthens diagnostics across the UI.

It also upgrades the project to .NET Framework 4.8.1, cleans up the WinForms implementation, and reorganizes supporting code and project structure.

Key changes

Discovery and replication

• Parallelized domain controller discovery and replication checks.
• Added progress updates during long-running discovery operations.
• Updated forest discovery so that, when DC override is enabled, discovery is routed through the user-selected domain controller instead of DNS-based discovery.
• Added guards for empty domain controller input.
• Marked DC-override globals as volatile to reduce stale-state risk across worker-thread usage.
• Added preflight checks and timeouts for unreachable DCs, solving Discovery can be slow in environments where DCs are offline or unreachable #3.

UI reliability

• Moved TreeListView updates to RunWorkerCompleted to avoid cross-thread UI access.
• Replaced ActiveForm usage with the current form instance.
• Fixed a TreeListView expansion crash by defining explicit AspectGetter and ChildrenGetter handlers.
• Used Close() where appropriate for more predictable dialog behavior.
• Improved dark-mode consistency across dialogs.

Diagnostics

• Updated the Ping dialog to support both IPv4 and IPv6.
• Cleaned up the Port Tester implementation.
• Added support for multi-port test runs.

Housekeeping

• Added basic unit tests for extracted business logic classes
• Extracted the AD domain controller object model into ADREPLDC.
• Migrated the project to .NET Framework 4.8.1.
• Enabled the WinForms out-of-process designer.
• Added .editorconfig.
• Simplified the folder layout.
• Updated README and resource files.
• Bumped the version to 1.4.0.0.
• Applied general code-style cleanup.

[zbalkan]

This PR became larger than I expected. I screwed up with parallelization and over-engineered and started from scratch, then quit. Thanks to LLMs, I get my old branches reviewed and merged them into this but it does a lot for a single PR.