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.cc

@@ -0,0 +1,46 @@
+#include "TreeSitterHighlighter.h"
+
+#ifdef KTE_ENABLE_TREESITTER
+
+#include "Buffer.h"
+#include <utility>
+
+namespace kte {
+
+TreeSitterHighlighter::TreeSitterHighlighter(const TSLanguage* lang, std::string filetype)
+    : language_(lang), filetype_(std::move(filetype))
+{
+}
+
+TreeSitterHighlighter::~TreeSitterHighlighter()
+{
+    disposeParser();
+}
+
+void TreeSitterHighlighter::ensureParsed(const Buffer& /*buf*/) const
+{
+    // Intentionally a stub to avoid pulling the Tree-sitter API and library by default.
+    // In future, when linking against tree-sitter, initialize parser_, set language_,
+    // and build tree_ from the buffer contents.
+}
+
+void TreeSitterHighlighter::disposeParser() const
+{
+    // Stub; nothing to dispose when not actually creating parser/tree
+}
+
+void TreeSitterHighlighter::HighlightLine(const Buffer &/*buf*/, int /*row*/, std::vector<HighlightSpan> &/*out*/) const
+{
+    // For now, no-op. When tree-sitter is wired, map nodes to TokenKind spans per line.
+}
+
+std::unique_ptr<LanguageHighlighter> CreateTreeSitterHighlighter(const char* filetype,
+                                                                 const void* (*get_lang)())
+{
+    const auto* lang = reinterpret_cast<const TSLanguage*>(get_lang ? get_lang() : nullptr);
+    return std::make_unique<TreeSitterHighlighter>(lang, filetype ? std::string(filetype) : std::string());
+}
+
+} // namespace kte
+
+#endif // KTE_ENABLE_TREESITTER