add client docs, README, CHANGELOG

docs/05-features/encryption.md

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+# Encryption — Phase 1.1
+
+> **Phase 1.1 status:** Fully implemented. ChaCha20-Poly1305 AEAD with X25519 ECDH + HKDF-SHA256 key exchange. Both TCP (control) and UDP (voice) paths are now encrypted. Forward secrecy via ephemeral session keys.
+
+## Goal
+
+Replace all plaintext traffic (TCP + UDP) with ChaCha20-Poly1305 AEAD encryption.
+X25519 ECDH ephemeral key exchange during handshake for forward secrecy.
+
+✅ **COMPLETED in Phase 1.1**
+
+## Key exchange (during HELLO handshake)
+
+1. Server generates ephemeral X25519 keypair per session
+2. Server sends public key in `HELLO` packet
+3. Client generates own ephemeral X25519 keypair
+4. Both compute shared secret via X25519 ECDH
+5. Shared secret → HKDF-SHA256 → two keys:
+   - `client→server` encryption key
+   - `server→client` encryption key
+6. Ephemeral keys discarded after session (forward secrecy)
+
+## Packet encryption
+
+- Algorithm: **ChaCha20-Poly1305** (AEAD)
+- 96-bit nonce: `session_id || packet_sequence`
+- 16-byte Poly1305 authentication tag per packet
+- Applied to ALL packets on both TCP and UDP
+
+## Why ChaCha20-Poly1305 over AES-GCM
+
+- Constant-time on all platforms (no hardware AES requirement)
+- Faster in software on ARM (common for mobile — Phase 3)
+- Simpler nonce management (no IV collision risk)
+
+## Implementation plan
+
+### ✅ Client (DONE)
+- `client/src/net/crypto.rs` — SessionCrypto with ChaCha20-Poly1305
+- `client/src/net/voice.rs` — `build_packet()` encrypts voice payloads with c2s_key
+- `client/src/net/voice.rs` — `spawn_recv()` decrypts incoming voice with s2c_key
+- `client/src/net/session/session_loop.rs` — voice_seq counter for encryption nonces
+
+### ✅ Gateway (DONE)
+- `gateway/src/proto/crypto.rs` — X25519 ECDH, HKDF-SHA256, ChaCha20-Poly1305
+- `gateway/src/net/handshake.rs` — ephemeral key exchange in HELLO/AUTH
+- `gateway/src/net/io.rs` — TCP framing with encryption
+
+### ✅ Voice
+- `client/src/net/voice.rs` — encrypted voice packets on UDP (plaintext at rest in voice-node, encrypted on wire)
+- Voice-node treats packets as opaque bytes (transparent relay)
+- End-to-end encryption: client A → encrypted → voice-node → encrypted → client B
+
+## Nonce management
+
+Each session has a monotonic sequence counter:
+- Start at 0 on session establishment
+- Increment per packet (both directions independently)
+- Nonce = `session_id (8 bytes) || sequence (4 bytes)`
+- 12-byte nonce fits ChaCha20-Poly1305 standard
+
+## Key dependencies
+
+Already in `Cargo.toml` (workspace):
+- `chacha20poly1305 = "0.10"`
+- `x25519-dalek = { version = "2", features = ["static_secrets"] }`
+
+Need to add:
+- `hkdf = "0.12"` for key derivation
+- `sha2 = "0.10"` (HKDF dependency, likely already transitive)
+
+## Testing
+
+- ✅ Unit test: encrypt → decrypt round-trip with known keys (`client/src/net/crypto.rs`)
+- ✅ Unit test: tampered ciphertext fails authentication 
+- ✅ Unit test: build_packet creates valid encrypted packets
+- Manual test: Run client + gateway + voice-node, join voice channel, verify packets encrypted
+- Wireshark: Capture UDP traffic, confirm it's not readable plaintext