Refactor syntax highlighting infrastructure and related classes.

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

syntax/ErlangHighlighter.cc

@@ -0,0 +1,159 @@
+#include "ErlangHighlighter.h"
+#include "../Buffer.h"
+#include <cctype>
+
+namespace kte {
+static void
+push(std::vector<HighlightSpan> &out, int a, int b, TokenKind k)
+{
+	if (b > a)
+		out.push_back({a, b, k});
+}
+
+
+static bool
+is_ident_start(char c)
+{
+	return std::isalpha(static_cast<unsigned char>(c)) || c == '_' || c == '\'';
+}
+
+
+static bool
+is_ident_char(char c)
+{
+	return std::isalnum(static_cast<unsigned char>(c)) || c == '_' || c == '@' || c == ':' || c == '?';
+}
+
+
+ErlangHighlighter::ErlangHighlighter()
+{
+	const char *kw[] = {
+		"after", "begin", "case", "catch", "cond", "div", "end", "fun", "if", "let", "of",
+		"receive", "when", "try", "rem", "and", "andalso", "orelse", "not", "band", "bor", "bxor",
+		"bnot", "xor", "module", "export", "import", "record", "define", "undef", "include", "include_lib"
+	};
+	for (auto s: kw)
+		kws_.insert(s);
+}
+
+
+void
+ErlangHighlighter::HighlightLine(const Buffer &buf, int row, std::vector<HighlightSpan> &out) const
+{
+	const auto &rows = buf.Rows();
+	if (row < 0 || static_cast<std::size_t>(row) >= rows.size())
+		return;
+	std::string s = static_cast<std::string>(rows[static_cast<std::size_t>(row)]);
+	int n         = static_cast<int>(s.size());
+	int i         = 0;
+
+	while (i < n) {
+		char c = s[i];
+		if (c == ' ' || c == '\t') {
+			int j = i + 1;
+			while (j < n && (s[j] == ' ' || s[j] == '\t'))
+				++j;
+			push(out, i, j, TokenKind::Whitespace);
+			i = j;
+			continue;
+		}
+		// comment
+		if (c == '%') {
+			push(out, i, n, TokenKind::Comment);
+			break;
+		}
+		// strings
+		if (c == '"') {
+			int j    = i + 1;
+			bool esc = false;
+			while (j < n) {
+				char d = s[j++];
+				if (esc) {
+					esc = false;
+					continue;
+				}
+				if (d == '\\') {
+					esc = true;
+					continue;
+				}
+				if (d == '"')
+					break;
+			}
+			push(out, i, j, TokenKind::String);
+			i = j;
+			continue;
+		}
+		// char literal $X
+		if (c == '$') {
+			int j = i + 1;
+			if (j < n && s[j] == '\\' && j + 1 < n)
+				j += 2;
+			else if (j < n)
+				++j;
+			push(out, i, j, TokenKind::Char);
+			i = j;
+			continue;
+		}
+		// numbers
+		if (std::isdigit(static_cast<unsigned char>(c))) {
+			int j = i + 1;
+			while (j < n && (std::isalnum(static_cast<unsigned char>(s[j])) || s[j] == '#' || s[j] == '.' ||
+			                 s[j] == '_'))
+				++j;
+			push(out, i, j, TokenKind::Number);
+			i = j;
+			continue;
+		}
+		// atoms/variables/identifiers (including quoted atoms)
+		if (is_ident_start(c)) {
+			// quoted atom: '...'
+			if (c == '\'') {
+				int j    = i + 1;
+				bool esc = false;
+				while (j < n) {
+					char d = s[j++];
+					if (d == '\'') {
+						if (j < n && s[j] == '\'') {
+							++j;
+							continue;
+						}
+						break;
+					}
+					if (d == '\\')
+						esc = !esc;
+				}
+				push(out, i, j, TokenKind::Identifier);
+				i = j;
+				continue;
+			}
+			int j = i + 1;
+			while (j < n && is_ident_char(s[j]))
+				++j;
+			std::string id = s.substr(i, j - i);
+			// lowercase leading -> atom/function/module; uppercase or '_' -> variable
+			TokenKind k = TokenKind::Identifier;
+			// keyword check (lowercase)
+			std::string lower;
+			lower.reserve(id.size());
+			for (char ch: id)
+				lower.push_back(static_cast<char>(std::tolower(static_cast<unsigned char>(ch))));
+			if (kws_.count(lower))
+				k = TokenKind::Keyword;
+			push(out, i, j, k);
+			i = j;
+			continue;
+		}
+		if (std::ispunct(static_cast<unsigned char>(c))) {
+			TokenKind k = TokenKind::Operator;
+			if (c == ',' || c == ';' || c == '(' || c == ')' || c == '[' || c == ']' || c == '{' || c ==
+			    '}')
+				k = TokenKind::Punctuation;
+			push(out, i, i + 1, k);
+			++i;
+			continue;
+		}
+		push(out, i, i + 1, TokenKind::Default);
+		++i;
+	}
+}
+} // namespace kte