docs: expand crypto primitives specification

[Kkkakania]

Closes #1

Summary

• Expand specs/crypto.md into a full formal specification for the five @anonvote/crypto primitives.
• Document algorithms, parameters, input/output formats, security properties, failure modes, and implementation references for each primitive.
• Add the required lifecycle dependency diagram, per-ballot key management guidance, and explicit threat-model boundaries.

Verification

• Cross-checked the function names and behavior against AnonVote/js/src/crypto.ts.
• Cross-checked the documented behaviors against AnonVote/js/tests/crypto.test.ts.
• Ran git diff --check.

Notes

• This repository is Markdown-only and has no package/build/test script to run.