Watch Protocol

message SessionMessage {
  oneof payload {
    StartSession      start_session       = 1;   // phone → watch
    StopSession       stop_session        = 2;   // phone → watch
    GetStatus         get_status          = 3;   // phone → watch
    SessionStarted    session_started     = 10;  // watch → phone
    SessionFrame      session_frame       = 11;  // watch → phone
    SessionSummary    session_summary     = 12;  // watch → phone
    SessionError      session_error       = 13;  // watch → phone
    SessionStatus     session_status      = 14;  // watch → phone (response to GetStatus)
    BiosignalBatch    biosignal_batch     = 15;  // watch → phone (raw streaming opt-in)
  }
}

message StartSession {
  string         session_id        = 1;
  Mode           mode              = 2;
  uint32         duration_sec      = 3;
  ComputeProfile profile           = 4;
  string         window_label      = 5;  // optional
  bool           enable_raw_stream = 6;  // default false
}

message ComputeProfile {
  uint32 window_sec        = 1;
  uint32 emit_interval_sec = 2;
}

message StopSession { string session_id = 1; }

message SessionStarted {
  string session_id    = 1;
  uint64 started_at_ms = 2;
}

message SessionFrame {
  string             session_id     = 1;
  uint64             seq            = 2;  // monotonic per session
  uint64             emitted_at_ms  = 3;
  SessionMetrics     metrics        = 4;
  BehaviorSnapshot   behavior       = 5;  // optional
}

message SessionMetrics {
  double hr_mean_bpm     = 1;
  double hr_sdnn_ms      = 2;
  double rmssd_ms        = 3;
  uint32 sample_count    = 4;
  uint64 start_ms        = 5;
  uint64 end_ms          = 6;

  optional double motion_rms_g          = 7;   // accel-equipped wearables
  optional uint32 motion_sample_count   = 8;
  optional double active_energy_kcal    = 9;   // iOS HKLiveWorkoutBuilder
}

message BehaviorSnapshot {
  double typing_cadence            = 1;
  double inter_key_latency         = 2;
  uint32 burst_length              = 3;
  double scroll_velocity           = 4;
  double scroll_acceleration       = 5;
  double scroll_jitter             = 6;
  double tap_rate                  = 7;
  uint32 app_switches_per_minute   = 8;
  double foreground_duration       = 9;
  double idle_gap_seconds          = 10;
  double stability_index           = 11;
  double fragmentation_index       = 12;
  uint64 timestamp                 = 13;
}

message SessionSummary {
  string           session_id          = 1;
  uint32           duration_actual_sec = 2;
  SessionMetrics   metrics             = 3;
  BehaviorSnapshot behavior            = 4;  // optional
}

message SessionError {
  string    session_id = 1;
  ErrorCode code       = 2;
  string    message    = 3;
}

message SessionStatus {
  string       session_id = 1;
  SessionState state      = 2;
  bool         active     = 3;
  uint64       last_seq   = 4;
}

message BiosignalSample {
  uint64     timestamp_ms = 1;
  SignalType type         = 2;   // SIGNAL_HR_BPM | SIGNAL_IBI_MS | SIGNAL_HRV_RMSSD
  float      value        = 3;
}

message BiosignalBatch {
  string                   session_id = 1;
  repeated BiosignalSample samples    = 2;
}

class SessionChannel {
  Future<void> startSession(SessionConfig config);
  Future<void> stopSession(String sessionId);
  Future<SessionStatus?> getStatus();
  Future<WatchStatus?> getWatchStatus();
  Stream<SessionEvent> get events;  // EventChannel-backed
}

class WatchStatus {
  final bool supported;   // OS supports watch companion
  final bool reachable;   // watch is reachable now
  final bool paired;      // a watch is paired
  final bool installed;   // companion app is installed on the watch
}