Add extensible highlighter registration and Tree-sitter support.

- Implemented runtime API for registering custom highlighters.
- Added optional Tree-sitter integration for advanced syntax parsing (disabled by default).
- Updated buffer initialization and copying to support dynamic highlighter configuration.
- Introduced `NullHighlighter` as a fallback for unsupported filetypes.
- Enhanced CMake configuration with `KTE_ENABLE_TREESITTER` option.

TreeSitterHighlighter.h

@@ -0,0 +1,48 @@
+// TreeSitterHighlighter.h - optional adapter for Tree-sitter (behind KTE_ENABLE_TREESITTER)
+#pragma once
+
+#ifdef KTE_ENABLE_TREESITTER
+
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "LanguageHighlighter.h"
+
+// Forward-declare Tree-sitter C API to avoid hard coupling in headers if includes are not present
+extern "C" {
+struct TSLanguage;
+struct TSParser;
+struct TSTree;
+}
+
+namespace kte {
+
+// A minimal adapter that uses Tree-sitter to parse the whole buffer and then, for now,
+// does very limited token classification. This acts as a scaffold for future richer
+// queries. If no queries are provided, it currently produces no spans (safe fallback).
+class TreeSitterHighlighter : public LanguageHighlighter {
+public:
+    explicit TreeSitterHighlighter(const TSLanguage* lang, std::string filetype);
+    ~TreeSitterHighlighter() override;
+
+    void HighlightLine(const Buffer &buf, int row, std::vector<HighlightSpan> &out) const override;
+
+private:
+    const TSLanguage* language_{nullptr};
+    std::string filetype_;
+    // Lazy parser to avoid startup cost; mutable to allow creation in const method
+    mutable TSParser* parser_{nullptr};
+    mutable TSTree* tree_{nullptr};
+
+    void ensureParsed(const Buffer& buf) const;
+    void disposeParser() const;
+};
+
+// Factory used by HighlighterRegistry when registering via RegisterTreeSitter.
+std::unique_ptr<LanguageHighlighter> CreateTreeSitterHighlighter(const char* filetype,
+                                                                 const void* (*get_lang)());
+
+} // namespace kte
+
+#endif // KTE_ENABLE_TREESITTER