Puzzle: unit tests for parsing and flow

[jasonappah]

Description
Add unit tests that validate puzzle parsing, validation, and basic play flow so regressions are caught early.

Why

• Prevent breaks in puzzle metadata parsing and turn/objective rules
• Document intended behavior through tests

Scope

• Tests for parsing puzzle metadata: side-to-move, objective, mate-in-N values
• Tests for happy path: load puzzle, make correct moves, assert completion
• Tests for key error paths: invalid metadata, illegal move, objective mismatch

Acceptance

• Tests run in CI and locally with one command
• Coverage touches parsing and core puzzle flow
• Failing behavior reproduces a clear error message