Fix validator listing and organize repository docs

[Bunda001]

Summary

• Add an ordered validator registry and paginated list_validators(start, limit) view, keeping the index in sync on registration and deregistration.
• Move root-level design and implementation documents into docs/design/ and docs/impl/, and update cross-references.
• Ignore full_structure3.txt and remove it from version control while preserving the local file.

Validation

• Local Markdown links and git diff --check pass.
• Could not run Rust tests because Cargo is unavailable in the environment.

Fixes #234
Fixes #248
Fixes #247

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