Introduce a plugin trait for custom puzzle types

[Mkalbani]

Description

All puzzle types are currently hardcoded into the core crate, making it hard for contributors to add new puzzle categories without touching central match statements. Introduce a Puzzle trait so new puzzle kinds can be implemented and registered without modifying core dispatch logic.

Acceptance Criteria

• Puzzle trait defined with generate(), check_answer(), hint(), and difficulty() methods
• Existing puzzle types refactored to implement the trait
• A PuzzleRegistry allows registering new puzzle implementations at startup
• At least one example third-party-style puzzle added under examples/ showing registration
• Existing puzzle behavior unchanged (verified by existing test suite passing)
• CONTRIBUTING.md updated with a short guide on adding a new puzzle type

[OluRemiFour]

I've reviewed the task and have a clear implementation plan. Looking forward to contributing with a quality PR.

[OluRemiFour]

I've reviewed the task and have a clear implementation plan. Looking forward to contributing with a quality PR.

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #27)
2. Your PR will be reviewed by the ThinknetCollective maintainers

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