Implemented core user profile management for DHT:

New Features:
- User profile data structures (dht_profile.h/c)
  - Profile with identity, nickname, pubkeys, avatar, groups
  - Nickname validation (3-32 chars, alphanumeric + underscore)
  - Binary serialization/deserialization
  - SHA256-based identity computation

- DHT storage functions (dna_dht.c)
  - dna_dht_register_user() - Register with optional nickname
  - dna_dht_lookup_profile() - Get profile by identity
  - dna_dht_lookup_nickname() - Resolve nickname → identity
  - dna_dht_update_profile() - Update profile (stub)

- Profile storage in DHT
  - Profile entry: SHA256("profile:identity") → user_profile_t
  - Nickname index: SHA256("nickname:alice") → identity
  - 1-year TTL for profiles
  - Nickname collision detection

- Python test client (test_client.py)
  - send - Send messages
  - inbox - Get message list
  - get - Retrieve specific message
  - register - Register user (ready for HTTP endpoint)
  - lookup - Lookup profile
  - search - Search by nickname
  - stats - Node statistics

- Demo script (demo.sh)
  - Quick testing of messaging APIs
  - Example usage patterns

Technical Details:
- Profile serialization: version + identity + flags + keys + metadata
- Supports groups array (for future group messaging)
- Avatar stored as hash reference (image stored separately)
- Backwards compatible with flat/nested config formats
- Thread-safe logging integrated

Testing:
✓ Profile creation and serialization
✓ Nickname validation
✓ Identity computation (SHA256 of pubkey)
✓ Clean build with no warnings

Still TODO:
- HTTP endpoints for /api/v1/users/* (Phase 2)
- Profile replication to peers
- Avatar image storage
- Profile update implementation

🤖 Generated with [Claude Code](https://claude.com/claude-code

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Co-Authored-By: Claude <noreply@anthropic.com>