Retire the Stately backend (#949)

Removes the Stately (state-diagram export) code generator, which is no
longer maintained. Deletes Src/PCompiler/CompilerCore/Backend/Stately/
and unwires it everywhere it was referenced:

- CompilerOutput: drop the Stately enum value.
- TargetLanguage: drop the Stately backend registration and using.
- PCompilerOptions: drop the 'stately' --mode case, the allowed value,
  and the help-text mention.
- ParsePProjectFile: drop the 'stately' project Target case and remove
  Stately from the expected-target error message.

No tutorials, tests, or CI jobs referenced Stately. Build and unit
tests pass.

Co-authored-by: Claude <noreply@anthropic.com>

Author: ankushdesai

Src/PCompiler/CompilerCore/Backend/Stately/CompilationContext.cs

@@ -2,7 +2,6 @@
 using Plang.Compiler.Backend.CSharp;
 using Plang.Compiler.Backend.Java;
 using Plang.Compiler.Backend.PEx;
-using Plang.Compiler.Backend.Stately;
 using Plang.Compiler.Backend.PVerifier;
 
 namespace Plang.Compiler.Backend
@@ -16,7 +15,6 @@ static TargetLanguage()
         {
             RegisterCodeGenerator(CompilerOutput.PChecker, new PCheckerCodeGenerator());
             RegisterCodeGenerator(CompilerOutput.PObserve, new PObserveCodeGenerator());
-            RegisterCodeGenerator(CompilerOutput.Stately, new StatelyCodeGenerator());
             RegisterCodeGenerator(CompilerOutput.PVerifier, new PVerifierCodeGenerator());
             RegisterCodeGenerator(CompilerOutput.PEx, new PExCodeGenerator());
         }

Src/PCompiler/CompilerCore/Backend/Stately/StatelyCodeGenerator.cs

@@ -4,7 +4,6 @@ public enum CompilerOutput
     {   
         PChecker,
         PObserve,
-        Stately,
         PEx,
         PVerifier
     }

Src/PCompiler/CompilerCore/Backend/TargetLanguage.cs

@@ -38,8 +38,8 @@ internal PCompilerOptions()
             pfilesGroup.AddArgument("projname", "pn", "Project name for the compiled output");
             pfilesGroup.AddArgument("outdir", "o", "Dump output to directory (absolute or relative path)");
 
-            var modes = Parser.AddArgument("mode", "md", "Compilation mode to use. Can be bugfinding, pex, pobserve, or stately. If this option is not passed, bugfinding mode is used as default");
-            modes.AllowedValues = new List<string>() { "bugfinding", "pex", "pobserve", "stately", "verification", "coverage" };
+            var modes = Parser.AddArgument("mode", "md", "Compilation mode to use. Can be bugfinding, pex, or pobserve. If this option is not passed, bugfinding mode is used as default");
+            modes.AllowedValues = new List<string>() { "bugfinding", "pex", "pobserve", "verification", "coverage" };
 
             Parser.AddArgument("pobserve-package", "po", "PObserve package name").IsHidden = true;
 
@@ -159,9 +159,6 @@ private static void UpdateConfigurationWithParsedArgument(CompilerConfiguration
                         case "pobserve":
                             compilerConfiguration.OutputLanguages.Add(CompilerOutput.PObserve);
                             break;
-                        case "stately":
-                            compilerConfiguration.OutputLanguages.Add(CompilerOutput.Stately);
-                            break;
                         default:
                             throw new Exception($"Unexpected mode: '{option.Value}'");
                     }

Src/PCompiler/CompilerCore/CompilerOutput.cs

@@ -231,15 +231,12 @@ private IList<CompilerOutput> GetTargetLanguages(FileInfo fullPathName)
                         case "pobserve":
                             outputLanguages.Add(CompilerOutput.PObserve);
                             break;
-                        case "stately":
-                            outputLanguages.Add(CompilerOutput.Stately);
-                            break;
                         case "pverifier":
                             outputLanguages.Add(CompilerOutput.PVerifier);
                             break;
                         default:
                             throw new CommandlineParsingError(
-                                $"Expected PChecker, PObserve, Stately, PVerifier, or Symbolic as target, received {projectXml.Element("Target")?.Value}");
+                                $"Expected PChecker, PObserve, PVerifier, or Symbolic as target, received {projectXml.Element("Target")?.Value}");
                     }
                 }
             }