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.

Highlight.h

@@ -0,0 +1,39 @@
+// Highlight.h - core syntax highlighting types for kte
+#pragma once
+
+#include <cstdint>
+#include <vector>
+
+namespace kte {
+
+// Token kinds shared between renderers and highlighters
+enum class TokenKind {
+    Default,
+    Keyword,
+    Type,
+    String,
+    Char,
+    Comment,
+    Number,
+    Preproc,
+    Constant,
+    Function,
+    Operator,
+    Punctuation,
+    Identifier,
+    Whitespace,
+    Error
+};
+
+struct HighlightSpan {
+    int col_start{0}; // inclusive, 0-based columns in buffer indices
+    int col_end{0};   // exclusive
+    TokenKind kind{TokenKind::Default};
+};
+
+struct LineHighlight {
+    std::vector<HighlightSpan> spans;
+    std::uint64_t version{0}; // buffer version used for this line
+};
+
+} // namespace kte