feat: P3-T3 bandwidth estimation — GCC-style congestion control

BandwidthEstimator tracks available bandwidth using dual signals:

Delay-based: EMA of RTT vs baseline minimum. If RTT > 1.5x baseline
→ Overuse (congestion). If RTT < 1.1x baseline → Underuse (headroom).
Baseline slowly drifts up to handle route changes.

Loss-based: sliding window of 10 loss samples. Average > 5% → congested.

Rate adaptation (AIMD):
- Overuse OR loss congested: decrease 15% (multiplicative)
- Underuse AND no loss: increase 5% (additive)
- Normal: hold steady
- Clamped to [min_bw, max_bw]

recommended_profile() maps bandwidth to quality tier:
- >= 25 kbps → GOOD (Opus 24k + 20% FEC)
- >= 8 kbps → DEGRADED (Opus 6k + 50% FEC)
- < 8 kbps → CATASTROPHIC (Codec2 1200 + 100% FEC)

from_quality_report() integrates with existing QualityReport packets.

54 proto tests passing (12 new bandwidth tests).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-proto/src/lib.rs

@@ -12,6 +12,7 @@
 //! - Identity = 32-byte seed → HKDF → Ed25519 (signing) + X25519 (encryption)
 //! - Fingerprint = SHA-256(Ed25519 pub)[:16]
 
+pub mod bandwidth;
 pub mod codec_id;
 pub mod error;
 pub mod jitter;
@@ -27,6 +28,7 @@ pub use packet::{
     HangupReason, MediaHeader, MediaPacket, MiniFrameContext, MiniHeader, QualityReport,
     SignalMessage, TrunkEntry, TrunkFrame, FRAME_TYPE_FULL, FRAME_TYPE_MINI,
 };
+pub use bandwidth::{BandwidthEstimator, CongestionState};
 pub use quality::{AdaptiveQualityController, Tier};
 pub use session::{Session, SessionEvent, SessionState};
 pub use traits::*;