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.

QtFrontend.cc

@@ -123,8 +123,7 @@ protected:
 		if (ed_ && viewport.height() > 0 && viewport.width() > 0) {
 			const Buffer *buf = ed_->CurrentBuffer();
 			if (buf) {
-				const auto &lines         = buf->Rows();
-				const std::size_t nrows   = lines.size();
+				const std::size_t nrows   = buf->Nrows();
 				const std::size_t rowoffs = buf->Rowoffs();
 				const std::size_t coloffs = buf->Coloffs();
 				const std::size_t cy      = buf->Cury();
@@ -144,9 +143,8 @@ protected:
 
 				// Iterate visible lines
 				for (std::size_t i = rowoffs, vis_idx = 0; i < last_row; ++i, ++vis_idx) {
-					// Materialize the Buffer::Line into a std::string for
-					// regex/iterator usage and general string ops.
-					const std::string line = static_cast<std::string>(lines[i]);
+					// Get line as string for regex/iterator usage and general string ops.
+					const std::string line = buf->GetLineString(i);
 					const int y            = viewport.y() + static_cast<int>(vis_idx) * line_h;
 					const int baseline     = y + fm.ascent();
 
@@ -982,4 +980,4 @@ GUIFrontend::Shutdown()
 		delete app_;
 		app_ = nullptr;
 	}
-}
+}