Named pool supervisor for pipeline tiers with dispatch: :pool.

What problem this solves

High-rate descended tiers (for example RTP parsing) can overload the connection process if run inline. :pool dispatch caps concurrent TierSession workers; when the pool is full, descends are dropped (at-most-once) and telemetry reports saturation.

Caps children at Config.tier_pool_size/0 unless :max_children is passed.

Internal note

Thin wrapper around TierSupervisor registered as TierSupervisor.Pool.

RFCs

• No STUN/TURN RFC; OTP supervision for bounded async tier workers