Migrate GerbilManager.sln to slnx format

- Generated GerbilManager.slnx via dotnet sln migrate (SDK 10.0.300)
- Removed legacy GerbilManager.sln after verified build
- Updated .vscode/tasks.json references to the new file

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.vscode/tasks.json

@@ -7,7 +7,7 @@
             "type": "process",
             "args": [
                 "build",
-                "${workspaceFolder}/GerbilManager.sln",
+                "${workspaceFolder}/GerbilManager.slnx",
                 "/property:GenerateFullPaths=true",
                 "/consoleloggerparameters:NoSummary;ForceNoAlign"
             ],
@@ -19,7 +19,7 @@
             "type": "process",
             "args": [
                 "publish",
-                "${workspaceFolder}/GerbilManager.sln",
+                "${workspaceFolder}/GerbilManager.slnx",
                 "/property:GenerateFullPaths=true",
                 "/consoleloggerparameters:NoSummary;ForceNoAlign"
             ],
@@ -33,7 +33,7 @@
                 "watch",
                 "run",
                 "--project",
-                "${workspaceFolder}/GerbilManager.sln"
+                "${workspaceFolder}/GerbilManager.slnx"
             ],
             "problemMatcher": "$msCompile"
         },