docs: design achievement fingerprint discovery

[Erilla]

Summary

• records the approved achievement-fingerprint discovery architecture
• defines privacy, queueing, budget, snapshot, and testing contracts
• adds the shared domain language context

Verification

• pnpm format:check
• pnpm lint
• pnpm typecheck
• pnpm test:unit (160 passed)
• pnpm test:integration (54 passed; Vitest completed in 59.11s, then the 60-second shell wrapper timed out)

[Erilla]

Superseded by #29, which contains this branch's three docs commits (2fefbe5, b805e87, 44670b7) and targets main directly. Closing to keep one merge path for the fingerprint work.