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.

HighlighterEngine.h

@@ -0,0 +1,45 @@
+// HighlighterEngine.h - caching layer for per-line highlights
+#pragma once
+
+#include <cstdint>
+#include <memory>
+#include <unordered_map>
+#include <vector>
+
+#include "Highlight.h"
+#include "LanguageHighlighter.h"
+
+class Buffer;
+
+namespace kte {
+
+class HighlighterEngine {
+public:
+    HighlighterEngine();
+    ~HighlighterEngine();
+
+    void SetHighlighter(std::unique_ptr<LanguageHighlighter> hl);
+
+    // Retrieve highlights for a given line and buffer version.
+    // If cache is stale, recompute using the current highlighter.
+    const LineHighlight &GetLine(const Buffer &buf, int row, std::uint64_t buf_version) const;
+
+    // Invalidate cached lines from row (inclusive)
+    void InvalidateFrom(int row);
+
+    bool HasHighlighter() const { return static_cast<bool>(hl_); }
+
+private:
+    std::unique_ptr<LanguageHighlighter> hl_;
+    // Simple cache by row index (mutable to allow caching in const GetLine)
+    mutable std::unordered_map<int, LineHighlight> cache_;
+    // For stateful highlighters, remember per-line state (state after finishing that row)
+    struct StateEntry {
+        std::uint64_t version{0};
+        // Using the interface type; forward-declare via header
+        StatefulHighlighter::LineState state;
+    };
+    mutable std::unordered_map<int, StateEntry> state_cache_;
+};
+
+} // namespace kte