Phase 3c: Classical editing mode (Steps 50-53)

Implement TextASTSync and TextEditor — the core text editing layer
that bridges raw text and the structured AST. TextASTSync provides
bidirectional text-AST sync with debounce via TreeSitterParser and
generators. TextEditor wraps a text buffer with edit operations,
undo/redo that tracks AST state, find/replace, and selection.

New files: TextASTSync.h, TextEditor.h, step50_test.cpp
Updated: step51/53 tests (replaced stubs with real includes),
CMakeLists.txt (tree-sitter linking), PROGRESS.md
Fixed: step53 find-position off-by-one, canRedo assert logic

19/19 tests pass (step50: 8, step51: 5, step53: 6)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

editor/src/TextASTSync.h

@@ -0,0 +1,69 @@
+#pragma once
+// Step 51: Text-AST bidirectional synchronization
+//
+// TextASTSync bridges raw text editing and the structured AST:
+//   setText() → stores text, marks parse pending (debounce)
+//   syncNow() → parses text via TreeSitterParser → updates AST
+//   getText() → regenerates text from current AST via generator
+//   getAST()  → returns current Module*
+//   setAST()  → replaces AST directly (structured edit path)
+
+#include "ast/Parser.h"
+#include "ast/Generator.h"
+#include <string>
+#include <memory>
+
+class TextASTSync {
+public:
+    void setText(const std::string& text, const std::string& language) {
+        text_ = text;
+        language_ = language;
+        parsePending_ = true;
+    }
+
+    Module* getAST() const {
+        return module_.get();
+    }
+
+    void setAST(std::unique_ptr<Module> module) {
+        module_ = std::move(module);
+        parsePending_ = false;
+    }
+
+    std::string getText() const {
+        if (!module_) return text_;
+        if (language_ == "python") {
+            PythonGenerator gen;
+            return gen.generate(module_.get());
+        } else if (language_ == "cpp") {
+            CppGenerator gen;
+            return gen.generate(module_.get());
+        } else if (language_ == "elisp") {
+            ElispGenerator gen;
+            return gen.generate(module_.get());
+        }
+        return text_;
+    }
+
+    void syncNow() {
+        if (!parsePending_) return;
+        if (language_ == "python") {
+            module_ = TreeSitterParser::parsePython(text_);
+        } else if (language_ == "cpp") {
+            module_ = TreeSitterParser::parseCpp(text_);
+        } else if (language_ == "elisp") {
+            module_ = TreeSitterParser::parseElisp(text_);
+        }
+        parsePending_ = false;
+    }
+
+    bool isParsePending() const { return parsePending_; }
+
+    const std::string& getLanguage() const { return language_; }
+
+private:
+    std::string text_;
+    std::string language_;
+    std::unique_ptr<Module> module_;
+    bool parsePending_ = false;
+};