feat: add TCP ICE candidate support (RFC 6544)

[getroot]

Summary

Add TCP ICE candidate (RFC 6544) support so that WebRTC clients can connect over TCP when UDP is unavailable or restricted.

Motivation

Some network environments block UDP traffic. TCP ICE candidates allow WebRTC sessions to fall back to TCP without requiring a TURN relay, reducing latency and infrastructure cost compared to relay.

Backward Compatibility

The existing OvenPlayer has IceTransportPolicy: 'relay' configured. This means the moment iceServers are delivered to the player, the browser exclusively uses TURN relay and ignores all direct candidates. This behavior is the foundation of the compatibility design.

Default behavior (TcpRelayForce=false, DefaultTransport=udptcp):

• Only direct candidates (UDP/TCP) are sent. No iceServers.
• Existing player: IceTransportPolicy=relay has no effect since no iceServers are provided. Connects via direct candidates normally.
• New player: same result.

TcpRelayForce=true:

• Only relay (iceServers) is sent. No direct candidates.
• Existing player: receives iceServers, IceTransportPolicy=relay kicks in, uses relay only.
• New player: same result.

?transport=all:

• UDP direct + TCP direct + relay (iceServers) are all sent.
• Existing player: receives iceServers, IceTransportPolicy=relay kicks in, uses relay only. Same behavior as before.
• New player (IceTransportPolicy=all): all three candidates available, browser picks by priority (UDP > TCP > relay). Maximum compatibility.

Configuration Changes

New <IceCandidates> fields:

<IceCandidates>
    <IceCandidate>*:10000/udp</IceCandidate>
    <IceWorkerCount>1</IceWorkerCount>            <!-- worker threads for UDP ICE candidate sockets (default: 1) -->
    <IceCandidate>*:10000/tcp</IceCandidate>      <!-- new: TCP direct ICE port (RFC 6544) -->
    <TcpIceWorkerCount>1</TcpIceWorkerCount>      <!-- new: worker threads for TCP ICE candidate sockets (default: 1) -->
    <TcpRelay>*:13478</TcpRelay>
    <TcpRelayWorkerCount>1</TcpRelayWorkerCount>  <!-- worker threads for TURN relay sockets (default: 1) -->
    
    <TcpRelayForce>false</TcpRelayForce>          <!-- replaces deprecated TcpForce (default: false) -->
    <DefaultTransport>udptcp</DefaultTransport>   <!-- new: transport policy when ?transport is absent (default: udptcp) -->
</IceCandidates>

<TcpForce> is preserved as a deprecated alias for <TcpRelayForce>.

Transport Policy (?transport)

Value	Direct candidates	Relay (iceServers)	Old player result	New player result
(none)	follows <DefaultTransport> (default: udptcp)	follows TcpRelayForce (default: false)	direct	direct
udp	UDP only	no	UDP direct	UDP direct
tcp	TCP only	no	TCP direct	TCP direct
relay	none	yes	relay	relay
udptcp	UDP + TCP	no	direct	direct
all	UDP + TCP	yes	relay only (IceTransportPolicy=relay)	UDP > TCP > relay

Known limitations

Work in progress. Testing ongoing.