Add protocol specification

PROTOCOL.md

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+# RootStream Protocol (v1)
+
+This document describes the on‑wire protocol used by RootStream for peer discovery, handshakes, and encrypted media/control transport.
+
+## Overview
+
+RootStream uses UDP with a small plaintext header and an encrypted payload. The encryption primitive is ChaCha20‑Poly1305 (IETF variant) and key agreement is X25519 derived from Ed25519 identity keys.
+
+Key properties:
+- UDP for low latency and drop‑tolerance
+- Small plaintext header for routing and replay protection
+- Authenticated encryption for all payloads after handshake
+
+## Packet Header (plaintext)
+
+All packets begin with a fixed header:
+
+```
+struct packet_header_t {
+  uint32_t magic;         // 0x524F4F54 ("ROOT")
+  uint8_t  version;       // 1
+  uint8_t  type;          // PKT_*
+  uint16_t flags;         // reserved (0)
+  uint64_t nonce;         // per-peer increasing nonce
+  uint16_t payload_size;  // encrypted payload size
+  uint8_t  mac[16];        // Poly1305 tag (from ciphertext)
+}
+```
+
+Notes:
+- `payload_size` is the size of the encrypted payload (ciphertext).
+- `mac` is produced by ChaCha20‑Poly1305 and validates ciphertext integrity.
+
+## Packet Types
+
+```
+PKT_HANDSHAKE = 0x01
+PKT_VIDEO     = 0x02
+PKT_AUDIO     = 0x03
+PKT_INPUT     = 0x04
+PKT_CONTROL   = 0x05
+PKT_PING      = 0x06
+PKT_PONG      = 0x07
+```
+
+## Handshake
+
+Handshake is unencrypted and establishes a shared session key.
+
+Payload format:
+```
+[32 bytes] Ed25519 public key
+[N bytes]  hostname (null‑terminated)
+```
+
+Flow:
+1. Client sends PKT_HANDSHAKE with its public key + hostname.
+2. Server derives X25519 shared secret from its Ed25519 private key and the client’s public key.
+3. Server responds with its own PKT_HANDSHAKE.
+4. Client derives the same shared secret.
+5. Both sides set `session.authenticated = true` and start encrypted traffic.
+
+## Encryption
+
+For all non‑handshake packets:
+- Payload is encrypted with ChaCha20‑Poly1305 (IETF).
+- Nonce is a per‑peer monotonically increasing 64‑bit counter stored in the header.
+
+## Video Payload (PKT_VIDEO)
+
+Video frames are fragmented into UDP‑sized chunks. The encrypted payload begins with a chunk header:
+
+```
+struct video_chunk_header_t {
+  uint32_t frame_id;
+  uint32_t total_size;
+  uint32_t offset;
+  uint16_t chunk_size;
+  uint16_t flags;         // reserved
+  uint64_t timestamp_us;  // capture timestamp
+}
+[chunk_size bytes] encoded video data
+```
+
+Reassembly:
+- Client reassembles chunks by `frame_id`.
+- Once `received >= total_size`, the full frame is passed to the decoder.
+
+## Audio Payload (PKT_AUDIO)
+
+Audio packets carry Opus data preceded by a small header:
+
+```
+struct audio_packet_header_t {
+  uint64_t timestamp_us;
+  uint32_t sample_rate;
+  uint16_t channels;
+  uint16_t samples;       // per channel
+}
+[N bytes] Opus packet
+```
+
+The client can use `timestamp_us` for A/V sync.
+
+## Input Payload (PKT_INPUT)
+
+Input events are serialized as:
+
+```
+struct input_event_pkt_t {
+  uint8_t  type;   // EV_KEY, EV_REL, etc.
+  uint16_t code;   // key/button code
+  int32_t  value;  // value/delta
+}
+```
+
+## Control Payload (PKT_CONTROL)
+
+Control packets carry a command and value:
+
+```
+struct control_packet_t {
+  uint8_t  cmd;    // CTRL_*
+  uint32_t value;  // command-specific
+}
+```
+
+Commands:
+```
+CTRL_PAUSE            0x01
+CTRL_RESUME           0x02
+CTRL_SET_BITRATE      0x03
+CTRL_SET_FPS          0x04
+CTRL_REQUEST_KEYFRAME 0x05
+CTRL_SET_QUALITY      0x06
+CTRL_DISCONNECT       0x07
+```
+
+## Keepalive
+
+- `PKT_PING` is sent periodically when connected.
+- `PKT_PONG` is a response to `PKT_PING`.
+
+## Versioning
+
+Protocol version is in the header. Compatibility rules:
+- Major version must match.
+- New fields should be appended to payloads with safe defaults.
+- Unknown packet types should be ignored.
+
+## Limits
+
+- Packet size targets MTU‑safe UDP payloads.
+- Maximum reassembled video frame size is bounded in code.
+
+## Reference
+
+See:
+- `include/rootstream.h` for struct definitions and constants.
+- `src/network.c` for handshake, encryption, fragmentation, and reassembly.