Refactor: support PowerShell 5.1 C# compilation & Pester legacy syntax compliance

.agents/AGENTS.md

@@ -17,6 +17,7 @@
   - Negation: use `Should Not BeNullOrEmpty`, `Should Not Throw`, `Should Not Be $null`
   - Boolean: use `Should Be $true` or `Should Be $false`
   - Comparison: use `Should BeGreaterThan 0`
+  - Collection Null Checks: Avoid piping empty collections (like `BlockingCollection` or arrays) directly to `Should Not Be $null`. PowerShell unpacks the empty collection to nothing, causing the check to fail. Instead, check using a parenthesized boolean assertion: `($null -ne $Engine.FileQueue) | Should Be $true`.
 
 ## 4. Re-Saving UTF-8 with BOM Safely
 - **Avoiding Encoding Corruption**: When converting/saving a file to UTF-8 with BOM via PowerShell, ensure you read the file using `-Encoding UTF8` before writing it back. Reading it as ANSI will corrupt existing Unicode glyphs:
@@ -52,3 +53,14 @@
 ## 8. Artifact and Workspace File Operations
 - **Artifact Directory Bounds**: Always write user-facing reports, plan documents, and walk-throughs in the designated conversation brain directory `C:\Users\ajjuk\.gemini\antigravity\brain\<conversation-id>/` and provide `ArtifactMetadata`.
 - **Workspace Files**: Never provide `ArtifactMetadata` when creating or modifying files inside the user's workspace directory (e.g. source files, `.vscode/settings.json`, `.cursorrules`).
+
+## 9. C# Inline Compilation Compatibility (PowerShell 5.1+)
+- **C# 5 Syntax Limits**: When defining inline C# class definitions via `Add-Type` in module files, restrict the code syntax to C# 5 or lower. Windows PowerShell 5.1 compiles code using the .NET 4.0 C# compiler, which does not support C# 6+ features (e.g., expression-bodied properties `=>`, string interpolation `$""`, null-conditional operator `?.`).
+- **Property Getters**: Use standard explicit getters instead of `=>`:
+  ```csharp
+  // Correct (C# 5):
+  public int ProcessedCount { get { return _processedCount; } }
+
+  // Incorrect (C# 6):
+  public int ProcessedCount => _processedCount;
+  ```

public/Start-DTAuditGui.ps1

@@ -0,0 +1,90 @@
+function Start-DTAuditGui {
+    [CmdletBinding()]
+    param(
+        [Parameter(Mandatory = $true)] [string]$ScanPath,
+        [Parameter(Mandatory = $true)] [string]$CsvLogPath,
+        [int]$QueueLimit = 10000
+    )
+
+    # 1. Instantiate the WPF window from file
+    Add-Type -AssemblyName PresentationFramework, PresentationCore, WindowsBase
+    $XamlPath = Join-Path $PSScriptRoot "..\src\UI\MainWindow.xaml"
+    [xml]$XamlContent = Get-Content -Raw -Path $XamlPath
+    $Reader = [System.Xml.XmlNodeReader]::new($XamlContent)
+    $Window = [System.Windows.Markup.XamlReader]::Load($Reader)
+
+    # 2. Extract UI control references
+    $StartButton = $Window.FindName("StartButton")
+    $ProgressBar = $Window.FindName("AuditProgressBar")
+    $StatusLabel = $Window.FindName("StatusText")
+    $PathLabel   = $Window.FindName("PathText")
+
+    $PathLabel.Text = "Target Path: $ScanPath"
+
+    # 3. Handle click event via asynchronous worker threads
+    $StartButton.Add_Click({
+        $StartButton.IsEnabled = $false
+
+        # Instantiate the shared state context using our precompiled C# engine
+        $Engine = [DriveTools.Core.AuditEngine]::new($QueueLimit, $CsvLogPath)
+        $LogicalCores = [System.Environment]::ProcessorCount
+
+        # Spawn consumer workers to compute hashes in parallel
+        [System.Threading.Tasks.Task]::Run({
+            $RunspacePool = [System.Management.Automation.Runspaces.RunspaceFactory]::CreateRunspacePool(1, $LogicalCores + 1)
+            $RunspacePool.Open()
+            $ActiveTasks = [System.Collections.Generic.List[object]]::new()
+
+            $WorkerBlock = { param([DriveTools.Core.AuditEngine]$EngineInstance) $EngineInstance.StartConsumerWorker() }
+
+            for ($i = 0; $i -lt $LogicalCores; $i++) {
+                $PS = [System.Management.Automation.PowerShell]::Create().AddScript($WorkerBlock).AddArgument($Engine)
+                $PS.RunspacePool = $RunspacePool
+                $AsyncResult = $PS.BeginInvoke()
+                $ActiveTasks.Add([PSCustomObject]@{ Pipeline = $PS; Result = $AsyncResult })
+            }
+
+            # File System Traversal (Producer Loop)
+            [System.Threading.Tasks.Task]::Run({
+                try {
+                    $Files = [System.IO.Directory]::EnumerateFiles($ScanPath, "*", [System.IO.SearchOption]::AllDirectories)
+                    foreach ($File in $Files) {
+                        if ([System.IO.File]::GetAttributes($File).HasFlag([System.IO.FileAttributes]::ReparsePoint)) { continue }
+                        $Engine.FileQueue.Add($File)
+                    }
+                }
+                finally { $Engine.FileQueue.CompleteAdding() }
+            })
+
+            # 4. Decoupled UI Progress Monitor Loop (Marshals to the UI Thread)
+            while (-not $Engine.FileQueue.IsCompleted) {
+                [System.Threading.Thread]::Sleep(250) # Throttles context switches to protect CPU frames
+
+                $Processed = $Engine.ProcessedCount
+                $CurrentPath = $Engine.ActiveFile
+                $DisplayPath = if ($CurrentPath.Length -gt 55) { "..." + $CurrentPath.Substring($CurrentPath.Length - 52) } else { $CurrentPath }
+
+                # Safe cross-thread invocation to avoid Thread Access Exceptions
+                [System.Windows.Application]::Current.Dispatcher.Invoke([Action]{
+                    $StatusLabel.Text = "Processing Data: $Processed files hashed..."
+                    $PathLabel.Text = "Current Node: $DisplayPath"
+                })
+            }
+
+            # Clean tear-down of pipeline contexts
+            foreach ($Task in $ActiveTasks) {
+                $null = $Task.Pipeline.EndInvoke($Task.Result)
+                $Task.Pipeline.Dispose()
+            }
+            $RunspacePool.Close(); $RunspacePool.Dispose()
+
+            [System.Windows.Application]::Current.Dispatcher.Invoke([Action]{
+                $StatusLabel.Text = "Audit Completed Successfully!"
+                $ProgressBar.Value = 100
+            })
+        })
+    })
+
+    # Render window frame model safely
+    $null = $Window.ShowDialog()
+}