feat: Add move comments functionality with full round-trip support

[ysgrProgramming]

Overview

This PR adds move comments functionality to the chess app, allowing users to add, edit, and remove comments for individual moves. Comments are preserved through export/import cycles.

TDD & Lint Verification

• I have written failing tests (Red) and confirmed make test fails for the new tests.
• I have implemented code to pass tests (Green) and confirmed make test passes.
• I have run make lint and fixed all errors.

All 266 tests passing.

Self-Walkthrough (Logical Reasoning)

Requirement (ID)	Implementation & Logic	Key File/Function
R1: Add comment field to Move	Extended Move interface with optional comment?: string field	src/lib/types.ts
R2: Update comment via UI	Added UPDATE_MOVE_COMMENT action to GameAction and reducer logic	src/lib/gameState.ts
R3: Display comments in MoveList	Added comment display UI with edit/add buttons in MoveList component	src/components/MoveList.tsx
R4: Export comments in kifu	Extended movesToText and movesToPGN to include comments in PGN-style {comment} format	src/lib/kifuExport.ts
R5: Import comments from kifu	Implemented parseKifuText function to parse kifu text and extract comments	src/lib/notation.ts
R6: Round-trip support	Comments are preserved through export → import cycles	src/lib/kifuExport.ts, src/lib/notation.ts
R7: Context API for comments	Added handleUpdateMoveComment to GameStateContext	src/contexts/GameStateContext.tsx

Decision Log

• Comment format: Chose PGN-style {comment} format for compatibility with standard chess notation tools.
• Comment escaping: Implemented escape/unescape logic for special characters ({}, quotes, newlines) in comments.
• Parsing approach: Used tokenization-based parser instead of complex regex to handle edge cases more reliably.
• UI design: Comments are displayed inline below moves, with edit/add buttons for each move. Editing uses a textarea with Enter key support.
• Backward compatibility: Comments are optional, so existing kifu without comments continue to work correctly.

Testing

• Added comprehensive unit tests for comment functionality in gameState.test.ts
• Added export/import tests in kifuExport.test.ts and notation.test.ts
• Added UI component tests in MoveList.test.tsx
• All tests passing (266 tests total)

Documentation

• Updated README.md with comment usage examples and feature description
• Added documentation for kifu import functionality