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.

[Menjay7]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. I’ll prioritize this immediately and ensure a reliable solution is delivered as soon as possible. Thank you!

[jaynomyaro]

Hi! I’m a full-stack developer with a strong blockchain background, and I’m confident I can resolve this issue quickly and effectively. I’ll deliver a clean, reliable solution with immediate attention to detail, ensuring it’s completed as soon as possible. Please assign it to me—I’m ready to get started right away.

[grantfox-oss]

🦊 GrantFox — @OluRemiFour has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

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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