feat(interceptor): add GCC sender-side bandwidth estimator

[nightness]

Summary

Add GccInterceptor — a Google Congestion Control (GCC, RFC 8698) bandwidth estimator that uses TWCC feedback to dynamically estimate available send bandwidth.

Key components:

• TrendlineFilter — sliding window (N=20) OLS linear regression on inter-packet delay gradients; dynamic overuse threshold [6ms, 600ms]
• AimdRateController — AIMD state machine: 1.08×/sec increase on Normal/Underusing, ×0.85 on Overuse; loss-based adjustment (> 10% loss → decrease)
• GccHandle — Arc<Mutex<GccShared>> output channel (no changes to framework's Eout = () contract)
• GccInterceptorBuilder::new() returns (builder, GccHandle) pair

Chain position (GCC must be inner to TwccSender):

TwccSender → GccInterceptor → SenderReport → NackResponder → NoopInterceptor

Usage:

let (gcc_builder, gcc_handle) = GccInterceptorBuilder::new();
let chain = Registry::new()
    .with(gcc_builder.build())
    .with(TwccSenderBuilder::new().build())
    .build();

// In app polling loop:
if let Some(bps) = gcc_handle.target_bitrate_bps() {
    encoder.set_bitrate(bps);
}

Also includes the JitterBuffer interceptor (receiver-side, RFC 3550 §A.8 adaptive playout delay) as a dependency.

Test plan

• cargo test -p rtc-interceptor gcc — 8 GCC tests
• cargo test -p rtc-interceptor jitter_buffer — 6 jitter buffer tests
• cargo test -p rtc-interceptor — all tests pass

🤖 Generated with Claude Code