docs: add evil-keys crate implementation plan
Comprehensive design document covering architecture, API surface, dispatch algorithm, integration steps, testing strategy with 106 test cases, and design decisions (shift normalization, conflict detection, count overflow). Ultraworked with [Sisyphus](https://github.com/code-yeongyu/claude-agent) Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
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