Fix MCP use-after-free, scanner chunk overlap, build scripts

- MCP bridge: guard against use-after-free when client disconnects
  during sendJson flush by re-checking m_client after write
- Scanner engine: fix chunk overlap advancing past region end on
  final chunk; fix fallback region flags for providers without
  enumerateRegions
- Build scripts: prefer GCC MinGW over LLVM-MinGW in PATH detection

scripts/build.ps1

@@ -283,9 +283,10 @@ function Find-MinGWDirectory {
     $toolsDir = Join-Path $qtRoot "Tools"
     
     if (Test-Path $toolsDir) {
+        # Prefer GCC-based MinGW (has g++.exe); exclude llvm-mingw. Prefer 64-bit, then newest.
         $mingwToolDirs = Get-ChildItem -Path $toolsDir -Directory -ErrorAction SilentlyContinue | Where-Object {
-            $_.Name -match 'mingw'
-        }
+            $_.Name -match '^mingw\d+_\d+$'
+        } | Sort-Object -Property @{ Expression = { if ($_.Name -match '_64$') { 1 } else { 0 } }; Descending = $true }, Name -Descending
         
         foreach ($dir in $mingwToolDirs) {
             $testBin = Join-Path $dir.FullName "bin\g++.exe"

scripts/build_qscintilla.ps1

@@ -318,10 +318,10 @@ $qtRoot = Split-Path (Split-Path $selectedQtDir -Parent) -Parent
 $toolsDir = Join-Path $qtRoot "Tools"
 
 if (Test-Path $toolsDir) {
-    # Look for MinGW tools directory
+    # Prefer GCC-based MinGW (has g++.exe); exclude llvm-mingw. Prefer 64-bit, then newest.
     $mingwToolDirs = Get-ChildItem -Path $toolsDir -Directory -ErrorAction SilentlyContinue | Where-Object {
-        $_.Name -match 'mingw'
-    }
+        $_.Name -match '^mingw\d+_\d+$'
+    } | Sort-Object -Property @{ Expression = { if ($_.Name -match '_64$') { 1 } else { 0 } }; Descending = $true }, Name -Descending
     
     foreach ($dir in $mingwToolDirs) {
         $testBin = Join-Path $dir.FullName "bin\g++.exe"