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.

MarkdownHighlighter.cc

@@ -0,0 +1,88 @@
+#include "MarkdownHighlighter.h"
+#include "Buffer.h"
+#include <cctype>
+
+namespace kte {
+
+static void push_span(std::vector<HighlightSpan> &out, int a, int b, TokenKind k) {
+    if (b > a) out.push_back({a,b,k});
+}
+
+void MarkdownHighlighter::HighlightLine(const Buffer &buf, int row, std::vector<HighlightSpan> &out) const
+{
+    LineState st; // not used in stateless entry
+    (void)HighlightLineStateful(buf, row, st, out);
+}
+
+StatefulHighlighter::LineState MarkdownHighlighter::HighlightLineStateful(const Buffer &buf, int row, const LineState &prev, std::vector<HighlightSpan> &out) const
+{
+    StatefulHighlighter::LineState state = prev;
+    const auto &rows = buf.Rows();
+    if (row < 0 || static_cast<std::size_t>(row) >= rows.size()) return state;
+    std::string s = static_cast<std::string>(rows[static_cast<std::size_t>(row)]);
+    int n = static_cast<int>(s.size());
+
+    // Reuse in_block_comment flag as "in fenced code" state.
+    if (state.in_block_comment) {
+        // If line contains closing fence ``` then close after it
+        auto pos = s.find("```");
+        if (pos == std::string::npos) {
+            push_span(out, 0, n, TokenKind::String);
+            state.in_block_comment = true;
+            return state;
+        } else {
+            int end = static_cast<int>(pos + 3);
+            push_span(out, 0, end, TokenKind::String);
+            // rest of line processed normally after fence
+            int i = end;
+            // whitespace
+            if (i < n) push_span(out, i, n, TokenKind::Default);
+            state.in_block_comment = false;
+            return state;
+        }
+    }
+
+    // Detect fenced code block start at beginning (allow leading spaces)
+    int bol = 0; while (bol < n && (s[bol]==' '||s[bol]=='\t')) ++bol;
+    if (bol + 3 <= n && s.compare(bol, 3, "```") == 0) {
+        push_span(out, bol, n, TokenKind::String);
+        state.in_block_comment = true; // enter fenced mode
+        return state;
+    }
+
+    // Headings: lines starting with 1-6 '#'
+    if (bol < n && s[bol] == '#') {
+        int j = bol; while (j < n && s[j] == '#') ++j; // hashes
+        // include following space and text as Keyword to stand out
+        push_span(out, bol, n, TokenKind::Keyword);
+        return state;
+    }
+
+    // Process inline: emphasis and code spans
+    int i = 0;
+    while (i < n) {
+        char c = s[i];
+        if (c == '`') {
+            int j = i + 1; while (j < n && s[j] != '`') ++j; if (j < n) ++j;
+            push_span(out, i, j, TokenKind::String); i = j; continue;
+        }
+        if (c == '*' || c == '_') {
+            // bold/italic markers: treat the marker and until next same marker as Type to highlight
+            char m = c; int j = i + 1; while (j < n && s[j] != m) ++j; if (j < n) ++j;
+            push_span(out, i, j, TokenKind::Type); i = j; continue;
+        }
+        // links []() minimal: treat [text](url) as Function
+        if (c == '[') {
+            int j = i + 1; while (j < n && s[j] != ']') ++j; if (j < n) ++j; // include ]
+            if (j < n && s[j] == '(') { while (j < n && s[j] != ')') ++j; if (j < n) ++j; }
+            push_span(out, i, j, TokenKind::Function); i = j; continue;
+        }
+        // whitespace
+        if (c == ' ' || c == '\t') { int j=i+1; while (j<n && (s[j]==' '||s[j]=='\t')) ++j; push_span(out, i, j, TokenKind::Whitespace); i=j; continue; }
+        // fallback: default single char
+        push_span(out, i, i+1, TokenKind::Default); ++i;
+    }
+    return state;
+}
+
+} // namespace kte