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.

UndoSystem.h

@@ -1,3 +1,44 @@
+/*
+ * UndoSystem.h - undo/redo system with tree-based branching
+ *
+ * UndoSystem manages the undo/redo history for a Buffer. It provides:
+ *
+ * - Tree-based undo: Multiple redo branches at each node (not just linear history)
+ * - Atomic grouping: Multiple operations can be undone/redone as a single step
+ * - Dirty tracking: Marks when buffer matches last saved state
+ * - Efficient storage: Nodes stored in UndoTree, operations applied to Buffer
+ *
+ * Key concepts:
+ *
+ * 1. Undo tree structure:
+ *    - Each edit creates a node in the tree
+ *    - Undo moves up the tree (toward root)
+ *    - Redo moves down the tree (toward leaves)
+ *    - Multiple redo branches preserved (not lost on new edits after undo)
+ *
+ * 2. Operation lifecycle:
+ *    - Begin(type): Start recording an operation (insert/delete)
+ *    - Append(text): Add content to the pending operation
+ *    - commit(): Finalize and add to undo tree
+ *    - discard_pending(): Cancel without recording
+ *
+ * 3. Atomic grouping:
+ *    - BeginGroup()/EndGroup(): Bracket multiple operations
+ *    - All operations in a group share the same group_id
+ *    - Undo/redo treats the entire group as one step
+ *
+ * 4. Integration with Buffer:
+ *    - UndoSystem holds a reference to its owning Buffer
+ *    - apply() executes undo/redo by calling Buffer's editing methods
+ *    - Buffer's dirty flag updated automatically
+ *
+ * Usage pattern:
+ *   undo_system.Begin(UndoType::Insert);
+ *   undo_system.Append("text");
+ *   undo_system.commit();  // Now undoable
+ *
+ * See also: UndoTree.h (storage), UndoNode.h (node structure)
+ */
 #pragma once
 #include <string_view>
 #include <cstddef>
@@ -77,4 +118,4 @@ private:
 
 	Buffer *buf_;
 	UndoTree &tree_;
-};
+};