Add syntax highlighting infrastructure

- Introduced `HighlighterRegistry` with support for multiple language highlighters (e.g., JSON, Markdown, Python).
- Added `JsonHighlighter` implementation for basic JSON syntax highlighting.

CppHighlighter.h

@@ -0,0 +1,34 @@
+// CppHighlighter.h - minimal stateless C/C++ line highlighter
+#pragma once
+
+#include <regex>
+#include <string>
+#include <unordered_set>
+#include <vector>
+
+#include "LanguageHighlighter.h"
+
+class Buffer;
+
+namespace kte {
+
+class CppHighlighter final : public StatefulHighlighter {
+public:
+    CppHighlighter();
+    ~CppHighlighter() override = default;
+
+    void HighlightLine(const Buffer &buf, int row, std::vector<HighlightSpan> &out) const override;
+    LineState HighlightLineStateful(const Buffer &buf,
+                                    int row,
+                                    const LineState &prev,
+                                    std::vector<HighlightSpan> &out) const override;
+
+private:
+    std::unordered_set<std::string> keywords_;
+    std::unordered_set<std::string> types_;
+
+    static bool is_ident_start(char c);
+    static bool is_ident_char(char c);
+};
+
+} // namespace kte