fix: Correct ToBytes method syntax and GPU declaration parsing

This commit fixes two critical bugs in the GPU code generation:

1. **ToBytes method used invalid syntax**: Changed from the incorrect
   type conversion syntax to using unsafe.Slice(), which is the
   modern and correct approach for Go 1.17+.

   Before (invalid):
   (*[unsafe.Sizeof(Type{})]byte)[unsafe.Pointer(s)]

   After (correct):
   unsafe.Slice((*byte)(unsafe.Pointer(s)), int(unsafe.Sizeof(*s)))

2. **Parser incorrectly matched regular structs as GPU structs**:
   GPUStructDecl and GPUFuncDecl used `@@*` (zero or more decorators),
   which caused regular structs/functions without decorators to be
   treated as GPU code.

   Changed both to `@@+` (one or more decorators required), ensuring
   only explicitly decorated code is treated as GPU/WGSL code.

This prevents calculator and other non-GPU examples from generating
unnecessary GPU files and fixes compilation errors.

Changes:
- Updated generateToBytesMethod to use unsafe.Slice
- Changed GPUStructDecl decorators from `@@*` to `@@+`
- Changed GPUFuncDecl decorators from `@@*` to `@@+`
- All tests passing
- Pre-commit checks passing

Author: claude

pkg/ast/gpu_ast.go

@@ -13,7 +13,7 @@ type GPUDecorator struct {
 // GPUStructDecl represents a struct marked with @gpu decorator
 type GPUStructDecl struct {
 	Pos        lexer.Position
-	Decorators []*GPUDecorator `@@*`
+	Decorators []*GPUDecorator `@@+`
 	Name       string          `"type" @Ident`
 	Struct     *GPUStructType  `@@`
 }
@@ -54,7 +54,7 @@ type GPUBindingDecl struct {
 // GPUFuncDecl represents a GPU shader function
 type GPUFuncDecl struct {
 	Pos        lexer.Position
-	Decorators []*GPUDecorator `@@*`
+	Decorators []*GPUDecorator `@@+`
 	Name       string          `"func" @Ident`
 	Params     []*GPUParameter `"(" (@@ ("," @@)*)? ")"`
 	Results    *GPUReturnType  `@@?`

pkg/codegen/gpu_go_generator.go

@@ -149,6 +149,10 @@ func (g *GPUGoGenerator) generateHelperMethods(gpuStruct *guixast.GPUStructDecl)
 }
 
 // Generate ToBytes method
+//
+//	Generates: func (s *StructName) ToBytes() []byte {
+//	    return unsafe.Slice((*byte)(unsafe.Pointer(s)), int(unsafe.Sizeof(*s)))
+//	}
 func (g *GPUGoGenerator) generateToBytesMethod(structName string) *ast.FuncDecl {
 	return &ast.FuncDecl{
 		Recv: &ast.FieldList{
@@ -175,32 +179,48 @@ func (g *GPUGoGenerator) generateToBytesMethod(structName string) *ast.FuncDecl
 			List: []ast.Stmt{
 				&ast.ReturnStmt{
 					Results: []ast.Expr{
-						&ast.IndexExpr{
-							X: &ast.ParenExpr{
-								X: &ast.StarExpr{
-									X: &ast.ArrayType{
-										Len: &ast.CallExpr{
+						// unsafe.Slice((*byte)(unsafe.Pointer(s)), int(unsafe.Sizeof(*s)))
+						&ast.CallExpr{
+							Fun: &ast.SelectorExpr{
+								X:   ast.NewIdent("unsafe"),
+								Sel: ast.NewIdent("Slice"),
+							},
+							Args: []ast.Expr{
+								// (*byte)(unsafe.Pointer(s))
+								&ast.CallExpr{
+									Fun: &ast.ParenExpr{
+										X: &ast.StarExpr{
+											X: ast.NewIdent("byte"),
+										},
+									},
+									Args: []ast.Expr{
+										&ast.CallExpr{
+											Fun: &ast.SelectorExpr{
+												X:   ast.NewIdent("unsafe"),
+												Sel: ast.NewIdent("Pointer"),
+											},
+											Args: []ast.Expr{ast.NewIdent("s")},
+										},
+									},
+								},
+								// int(unsafe.Sizeof(*s))
+								&ast.CallExpr{
+									Fun: ast.NewIdent("int"),
+									Args: []ast.Expr{
+										&ast.CallExpr{
 											Fun: &ast.SelectorExpr{
 												X:   ast.NewIdent("unsafe"),
 												Sel: ast.NewIdent("Sizeof"),
 											},
 											Args: []ast.Expr{
-												&ast.CompositeLit{
-													Type: ast.NewIdent(structName),
+												&ast.StarExpr{
+													X: ast.NewIdent("s"),
 												},
 											},
 										},
-										Elt: ast.NewIdent("byte"),
 									},
 								},
 							},
-							Index: &ast.CallExpr{
-								Fun: &ast.SelectorExpr{
-									X:   ast.NewIdent("unsafe"),
-									Sel: ast.NewIdent("Pointer"),
-								},
-								Args: []ast.Expr{ast.NewIdent("s")},
-							},
 						},
 					},
 				},