OTP application root. Starts the long-lived infrastructure that every Session depends on (see docs/adr/ for rationale):

• Finch — HTTP/2 pool for the Provider (ADR 0002/0003).
• Pixir.Events.Registry — the event bus dispatch table (ADR 0004).
• Pixir.Sessions.Registry — session_id → Session pid (ADR 0001).
• Pixir.TurnSupervisor — a Task.Supervisor that owns each Turn's Task; the Session interrupts a Turn by killing its Task (ADR 0001).
• Pixir.Auth — owns the Credential and serializes token refresh (ADR 0002).
• Pixir.Provider.ConnectionSupervisor — owns per-key WebSocket Provider connections for the WebSocket-first transport policy (ADR 0019).
• Pixir.SessionSupervisor — a DynamicSupervisor of Pixir.Session processes.
• Pixir.Subagents — supervises parent-led Subagent fan-out (ADR 0011).