Refactor syntax highlighting infrastructure and related classes.

- Moved all language highlighter implementations (`CppHighlighter`, `GoHighlighter`, `JsonHighlighter`, etc.), the engine, and registry to `syntax/`.

syntax/LanguageHighlighter.h

@@ -0,0 +1,51 @@
+// LanguageHighlighter.h - interface for line-based highlighters
+#pragma once
+
+#include <memory>
+#include <vector>
+#include <string>
+
+#include "../Highlight.h"
+
+class Buffer;
+
+namespace kte {
+class LanguageHighlighter {
+public:
+	virtual ~LanguageHighlighter() = default;
+
+	// Produce highlight spans for a given buffer row. Implementations should append to out.
+	virtual void HighlightLine(const Buffer &buf, int row, std::vector<HighlightSpan> &out) const = 0;
+
+
+	virtual bool Stateful() const
+	{
+		return false;
+	}
+};
+
+// Optional extension for stateful highlighters (e.g., multi-line comments/strings).
+// Engines may detect and use this via dynamic_cast without breaking stateless impls.
+class StatefulHighlighter : public LanguageHighlighter {
+public:
+	struct LineState {
+		bool in_block_comment{false};
+		bool in_raw_string{false};
+		// For raw strings, remember the delimiter between the opening R"delim( and closing )delim"
+		std::string raw_delim;
+	};
+
+	// Highlight one line given the previous line state; return the resulting state after this line.
+	// Implementations should append spans for this line to out and compute the next state.
+	virtual LineState HighlightLineStateful(const Buffer &buf,
+	                                        int row,
+	                                        const LineState &prev,
+	                                        std::vector<HighlightSpan> &out) const = 0;
+
+
+	bool Stateful() const override
+	{
+		return true;
+	}
+};
+} // namespace kte