Implement EventStream — consume full SDK event stream, fan out to handlers

[infraax]

Task: EventStream

Parent epic: #4 (M1 — Robot Connection)
Depends on: #16 (ConnectionManager)

What to do

Create internal/brain/event_stream.go — subscribes to the vector-go-sdk's full event stream and fans each event out to registered handlers (the event bridge, state updater, etc.).

type EventHandler func(ctx context.Context, esn string, event *vectorpb.Event)

type EventStream struct { ... }

func NewEventStream(cm *ConnectionManager) *EventStream
func (es *EventStream) Register(handler EventHandler)
func (es *EventStream) Start(ctx context.Context) error  // blocking, call in goroutine

Event types to handle (initial set)

• RobotState — battery, pose, status flags
• RobotObservedFace — face detected with ID and name
• RobotObservedObject — cube or custom object
• StimulationInfo — internal stimulation level
• WakeWord — wake word detected
• AttentionTransfer — attention shifted
• HeldFillerAudio — robot is "thinking"

Non-blocking contract

The event goroutine must never block. If a handler is slow, it must run in its own goroutine or drop the event. The SDK stream goroutine is sacred — blocking it means missing events.

Notes

• Start the EventStream in a goroutine from ConnectionManager.Start().
• On SDK connection loss, Start() should return — the reconnect loop in ConnectionManager will restart it.
• Log unknown event types at DEBUG level (we'll encounter firmware events we haven't seen yet).

Definition of done

EventStream running, at least RobotState events arriving and updating ConnectionManager internal state. Verified with robot connected.