Step 2: HSDir descriptor fetching

[igor53627]

Parent Issue

Part of #23 (Onion service support)

Overview

Implement fetching of hidden service descriptors from HSDir relays.

Tasks

• Compute HSDir responsible relays from consensus
• Implement blinded key derivation using tor-hscrypto
• Build HSv3 descriptor request (GET /tor/hs/3/...)
• Fetch descriptor over existing Tor circuit infrastructure
• Handle descriptor caching (optional for MVP)

Technical Notes

• HSDir selection requires consensus data (already available)
• Blinding uses time periods from tor-hscrypto::time
• Descriptor is fetched via directory protocol over Tor

Dependencies

• Step 1: Onion address parsing and validation #37 (Onion address parsing)

Acceptance Criteria

• Can fetch HS descriptor for known .onion service
• Handles HSDir failures gracefully (retry different HSDir)