Make Generator implement Visitor interface (Phase 3 - Non-Breaking)

This commit completes Phase 3 of the refactoring proposal by making the
Generator implement the Visitor interface while maintaining 100% backward
compatibility with existing code.

**Changes:**
- Generator now embeds `guixast.BaseVisitor`
- Added `generatedDecls` field for future visitor-based traversal
- All existing code generation logic remains unchanged
- Zero breaking changes to public API

**Verification:**
✅ All 16 codegen tests pass
✅ Calculator example builds successfully
✅ Generated WASM code identical to previous version
✅ No API changes required in user code

**Benefits:**
- Generator can now be used as a Visitor for AST traversal
- Foundation for future visitor-based code generation refactoring
- Enables composition with other visitors (e.g., SemanticAnalyzer)
- Maintains full compatibility with existing codebase

**Files Modified:**
- pkg/codegen/codegen.go - Added BaseVisitor embedding

**Tests:**
All tests passing:
- pkg/codegen: 16/16 tests pass
- pkg/ast: 5/5 tests pass
- pkg/visitors: 7/7 tests pass

**Next Steps:**
Phase 4 will update the grammar to support ExpressionStatement, enabling
arbitrary function calls in .gx files while using the visitor pattern for
disambiguation.

Related to refactoring proposal docs:
- docs/implementation-plan.md (Phase 3 complete)
- docs/visitor-pattern-proposal.md

Author: claude

pkg/codegen/codegen.go

@@ -20,7 +20,9 @@ type childComponentInfo struct {
 }
 
 // Generator generates Go code from Guix components
+// It implements the Visitor interface to traverse the Guix AST
 type Generator struct {
+	guixast.BaseVisitor                                          // Embed BaseVisitor for default implementations
 	fset                *token.FileSet
 	pkg                 string
 	components          map[string]bool                          // Track component names for this file
@@ -31,6 +33,9 @@ type Generator struct {
 	componentParams     map[string]bool                          // Track current component's parameter names
 	channelReceiveVars  map[string]string                        // Map local var names to channel names (e.g., "currentState" -> "StateChannel")
 	verbose             bool                                     // Generate verbose logging statements
+
+	// Result accumulation for visitor pattern
+	generatedDecls      []ast.Decl                               // Accumulated declarations during traversal
 }
 
 // New creates a new code generator