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.

HighlighterRegistry.h

@@ -1,10 +1,11 @@
-// HighlighterRegistry.h - create/detect language highlighters
+// HighlighterRegistry.h - create/detect language highlighters and allow external registration
 #pragma once
 
 #include <functional>
 #include <memory>
 #include <string>
 #include <string_view>
+#include <vector>
 
 #include "LanguageHighlighter.h"
 
@@ -12,6 +13,8 @@ namespace kte {
 
 class HighlighterRegistry {
 public:
+    using Factory = std::function<std::unique_ptr<LanguageHighlighter>()>;
+
     // Create a highlighter for normalized filetype id (e.g., "cpp", "json", "markdown", "shell", "go", "python", "rust", "lisp").
     static std::unique_ptr<LanguageHighlighter> CreateFor(std::string_view filetype);
 
@@ -21,6 +24,26 @@ public:
 
     // Normalize various aliases/extensions to canonical ids.
     static std::string Normalize(std::string_view ft);
+
+    // Extensibility: allow external code to register highlighters at runtime.
+    // The filetype key is normalized via Normalize(). If a factory is already registered for the
+    // normalized key and override=false, the existing factory is kept.
+    static void Register(std::string_view filetype, Factory factory, bool override_existing = true);
+
+    // Returns true if a factory is registered for the (normalized) filetype.
+    static bool IsRegistered(std::string_view filetype);
+
+    // Return a list of currently registered (normalized) filetypes. Primarily for diagnostics/tests.
+    static std::vector<std::string> RegisteredFiletypes();
+
+#ifdef KTE_ENABLE_TREESITTER
+    // Forward declaration to avoid hard dependency when disabled.
+    struct TSLanguage;
+    // Convenience: register a Tree-sitter-backed highlighter for a filetype.
+    // The getter should return a non-null language pointer for the grammar.
+    static void RegisterTreeSitter(std::string_view filetype,
+                                   const TSLanguage* (*get_language)());
+#endif
 };
 
 } // namespace kte