Add benchmarks, migration tests, and dev guide

Add benchmarks for core operations, migration edge case tests, improved
buffer I/O tests, and developer guide

- Introduced `test_benchmarks.cc` for performance benchmarking of key
  operations in `PieceTable` and `Buffer`, including syntax highlighting
  and iteration patterns.
- Added `test_migration_coverage.cc` to provide comprehensive tests for
  migration of `Buffer::Rows()` to `PieceTable` APIs, with edge cases,
  boundary handling, and consistency checks.
- Enhanced `test_buffer_io.cc` with additional cases for save/load
  workflows, file handling, and better integration with the core API.
- Documented architectural details and core concepts in a new
  `DEVELOPER_GUIDE.md`. Highlighted design principles, code
  organization, and contribution workflows.

syntax/JsonHighlighter.cc

@@ -13,10 +13,9 @@ is_digit(char c)
 void
 JSONHighlighter::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())
+	if (row < 0 || static_cast<std::size_t>(row) >= buf.Nrows())
 		return;
-	std::string s = static_cast<std::string>(rows[static_cast<std::size_t>(row)]);
+	std::string s = buf.GetLineString(static_cast<std::size_t>(row));
 	int n         = static_cast<int>(s.size());
 	auto push     = [&](int a, int b, TokenKind k) {
 		if (b > a)
@@ -87,4 +86,4 @@ JSONHighlighter::HighlightLine(const Buffer &buf, int row, std::vector<Highlight
 		++i;
 	}
 }
-} // namespace kte
+} // namespace kte