docs: add crypto package integration guide

[Kkkakania]

Summary

• Add specs/crypto-integration.md as a formal integration guide for @anonvote/crypto.
• Document correct composition for identity hashing, token lifecycle, and vote encryption/tally flows.
• Specify per-ballot key generation, storage, blast-radius, and rotation requirements.
• Document six misuse patterns grounded in the current AnonVote/core implementation.
• Link the new guide from the protocol README contents table.

Verification

• Cross-checked primitive behavior against AnonVote/js/src/crypto.ts.
• Cross-checked integration risks against AnonVote/core services, config, and Prisma schema.
• Ran npm test in AnonVote/js: 15 tests passed.
• Ran npm run build in AnonVote/js.
• Ran a package smoke test covering the guide's core examples against dist/index.js.
• Ran git diff --check.

Closes #4