Rounds 4-6: peek fix, mutable lambda, isDefault/isDelete, pack expansion

Critical bug fix:
- peek() was mutating pos via skipCommentsAt(), causing wrong token
  visibility when comments separated tokens from checkKeyword calls

Parser:
- consteval/constinit added to keyword set and qualifier loops
- Explicit template instantiation: template class Foo<int>;
- Concept-constrained template params: Numeric T
- Global structured binding at top level
- Bit field declarations: unsigned int x : 1
- __attribute__((...)): consumed in trailing qualifier position
- Args&&... variadic params: isVariadic set from type-level pack expansion
- args... pack expansion in function call arguments -> PostfixExpr
- Local struct definition: emits via CodeGen.generateNode
- isDefault/isDelete properly set in parseFunctionOrConstructorOrDestructor
- mutable lambda: stored in LambdaExpr.isMutable

AstDecl/AstExpr:
- FunctionDecl.isDefault, FunctionDecl.isDelete
- LambdaExpr.isMutable

CodeGen:
- Emit = default / = delete for function declarations
- Emit mutable after lambda params, before ->
- Trailing return type: auto name(params) -> decltype(...)

CppBuild:
- TopLevelStatements hoisted to namespace scope (structured bindings)
- Explicit template instantiations emitted before namespace Processing

Author: pepc84

mode/CppMode.jar

@@ -379,6 +379,7 @@ record LambdaExpr(
     List<Capture> captures,
     List<Param> params,
     TypeRef returnType,
+    boolean isMutable,
     Block body,
     int line,
     int col,

src/java/AstExpr.java

@@ -979,6 +979,7 @@ public static String renderExpr(Expr e) {
                 if (p.name() != null) sb.append(' ').append(p.name());
             }
             sb.append(")");
+            if (l.isMutable()) sb.append(" mutable");
             if (l.returnType() != null) sb.append(" -> ").append(renderTypeRef(l.returnType()));
             sb.append(" ");
             StringBuilder bodySb = new StringBuilder();

src/java/CodeGen.java

@@ -1231,6 +1231,9 @@ private File writeSketchImpl(RunnerListener listener) throws IOException {
           boolean isDeductionGuide = rt.contains("->") && rt.contains("(") && !rt.startsWith("#");
           if (isTypeAlias || isDeductionGuide) {
             deferredAliases.add(item);
+          } else if (rt.startsWith("template ") && !rt.startsWith("template<") && !rt.startsWith("template <")) {
+            // Explicit template instantiation: must be outside namespace Processing
+            preNs.append(CodeGen.generateNode(item, 0));
           } else {
             out.append(CodeGen.generateNode(item, 0));
           }
@@ -1331,8 +1334,19 @@ private File writeSketchImpl(RunnerListener listener) throws IOException {
         && definedFunctions.contains(fd.name() + "/" + fd.params().size())
       );
 
-      out.append("struct Sketch : public PApplet {\n");
+      // Hoist TopLevelStatements (structured bindings, explicit instantiations) to namespace scope
+      List<TopLevelItem> nsHoisted = new ArrayList<>();
+      List<TopLevelItem> realMembers = new ArrayList<>();
       for (TopLevelItem item : sketchMembers) {
+        if (item instanceof TopLevelStatement) {
+          nsHoisted.add(item);
+        } else {
+          realMembers.add(item);
+        }
+      }
+      for (TopLevelItem item : nsHoisted) out.append(CodeGen.generateNode(item, 0));
+      out.append("struct Sketch : public PApplet {\n");
+      for (TopLevelItem item : realMembers) {
         out.append(CodeGen.generateNode(item, 1));
       }
       out.append("};\n");

src/java/CppBuild.java

@@ -922,9 +922,9 @@ private FunctionDecl parseFunctionOrConstructorOrDestructor(List<CppLexerToken>
             int save = pos;
             advance();
             if (checkKeyword("default")) {
-                advance(); // = default
+                advance(); isDefault = true;
             } else if (checkKeyword("delete")) {
-                advance(); // = delete
+                advance(); isDelete = true;
             } else if (checkLiteralZero()) {
                 advance();
                 isPureVirtual = true;
@@ -2274,12 +2274,14 @@ private List<Expr> parseArgList() {
         expectPunct("(");
         List<Expr> args = new ArrayList<>();
         if (!checkPunct(")")) {
-            args.add(parseExpr());
-            matchPunct("..."); // pack expansion: "f(args...)"
+            Expr a0 = parseExpr();
+            if (matchPunct("...")) a0 = new PostfixExpr("...", a0, a0.line(), a0.col(), List.of());
+            args.add(a0);
             while (matchPunct(",")) {
                 if (checkPunct(")")) break;
-                args.add(parseExpr());
-                matchPunct("..."); // pack expansion
+                Expr ai = parseExpr();
+                if (matchPunct("...")) ai = new PostfixExpr("...", ai, ai.line(), ai.col(), List.of());
+                args.add(ai);
             }
         }
         expectPunct(")");
@@ -2671,7 +2673,7 @@ private Expr parseLambda() {
         }
 
         // mutable, noexcept, attributes
-        matchKeyword("mutable");
+        boolean isMutable = matchKeyword("mutable");
         if (checkKeyword("noexcept")) { advance(); if(checkPunct("(")){advance();int _d=1;while(!isAtEnd()&&_d>0){if(checkPunct("("))_d++;else if(checkPunct(")"))_d--;advance();}} }
         consumeAttributes();
 
@@ -2681,7 +2683,7 @@ private Expr parseLambda() {
         }
 
         Block body = parseBlock();
-        return new LambdaExpr(captures, params, returnType, body, start.line(), start.col(), List.of());
+        return new LambdaExpr(captures, params, returnType, isMutable, body, start.line(), start.col(), List.of());
     }
 
     private Capture parseCapture() {
@@ -2749,7 +2751,7 @@ private Param parseParam() {
         }
 
         TypeRef type = parseTypeRef();
-        matchPunct("..."); // pack expansion after type: "Rest... rest"
+        boolean isTypePackExpansion = matchPunct("..."); // pack expansion after type: "Rest... rest"
 
         // Member pointer parameter: "int Widget::* dp" or "int (Widget::* mp)(int) const"
         if (check(CppLexerTokenType.IDENTIFIER) && pos + 1 < tokens.size()
@@ -2825,7 +2827,7 @@ private Param parseParam() {
         }
 
         // Variadic pack expansion after the type: "Args... args" or "Ts&&... ts"
-        boolean isVariadic = false;
+        boolean isVariadic = isTypePackExpansion;
         if (checkPunct("...")) { advance(); isVariadic = true; }
 
         String name;
@@ -2936,11 +2938,19 @@ private List<Statement> parseStatementOrMultiDecl(List<CppLexerToken> leadingCom
                     || tokens.get(pos + 1).isPunct("{"))) {
             TypeDef td = parseTypeDef(leadingComments, List.of());
             matchPunct(";");
-            // If a variable declaration follows, parse it
+            // Emit struct as verbatim text via CodeGen
+            String structCode = processing.mode.cpp.CodeGen.generateNode(td, 0).stripTrailing();
+            ExprStatement structStmt = new ExprStatement(
+                new Identifier(structCode, td.line(), td.col(), List.of()),
+                td.line(), td.col(), leadingComments);
+            // If a variable declaration follows, parse it too
             if (looksLikeDeclaration()) {
-                return new java.util.ArrayList<>(parseDeclStatementsDesugared(List.of()));
+                List<Statement> result = new java.util.ArrayList<>();
+                result.add(structStmt);
+                result.addAll(parseDeclStatementsDesugared(List.of()));
+                return result;
             }
-            return List.of(new ExprStatement(new Identifier("", td.line(), td.col(), List.of()), td.line(), td.col(), leadingComments));
+            return List.of(structStmt);
         }
         if (looksLikeDeclaration()) {
             return new ArrayList<Statement>(parseDeclStatementsDesugared(leadingComments));