perf: per-connection channels for server (exit proxy)

[Yanstart]

Context

Design: docs/plans/2026-03-19-channel-based-io-driver-design.md — Phase 2/3

Problem: Server has global Mutex<HashMap<ConnectionId, QuicConnection>>. ALL connections contend on ONE lock. stream_send() locks connections + batched_sender. run() loop also needs connections for handle_packet + flush_all_batched.

Solution

Per-connection mpsc channels for stream_send. run() loop remains UDP socket owner and packet demuxer. Per-connection flush tasks drain channels.

Changes Required

crates/quick-transport/src/server.rs

• Add per-connection channels: conn_channels: HashMap<ConnectionId, mpsc::Sender<ConnCommand>>
• Each new connection gets its own bounded channel
• stream_send() routes through per-connection channel (no global lock)
• run() loop demuxes packets, calls conn.recv() with brief per-connection lock
• Per-connection task handles send commands + flush

bins/quick-exit/src/tunnel.rs

• Adapt to new server.stream_send() API (should be transparent — same signature)

Acceptance Criteria

• Per-connection channels replace global Mutex for stream_send
• run() loop still owns UDP socket recv
• 351+ tests pass, CI green
• Proxmox benchmark: terrestrial download >40 Mbps, LEO download >7 Mbps

Dependencies

Requires #116 (client I/O driver) validated first.

Expected Impact

Symmetrical improvement for exit→entry (download) direction. Combined with #116: 75 Mbps terrestrial target.