perf: channel-based I/O driver for client (upload fix)

[Yanstart]

Context

Design: docs/plans/2026-03-19-channel-based-io-driver-design.md — Phase 1/3

Problem: After stages 1-4, upload stuck at ~8 Mbps on 95 Mbps baseline. stream_send() and poll task share the same Mutex. They serialize each other regardless of the event-driven backpressure changes.

Current: Download improved +135% (15 Mbps) and +232% (LEO 11.8 Mbps) but upload unaffected.

Solution

Replace Mutex with a channel-based I/O driver. Single tokio task owns quiche Connection exclusively. Relay tasks communicate via bounded mpsc channel (capacity 64 = 4MB).

Changes Required

crates/quick-transport/src/client.rs

• Add SendCommand enum (StreamSend, StreamRecv, OpenStream, DgramSend, GetStats, Close)
• Add run_io_driver() async function (select! loop: channel + socket + timer)
• Add flush_packets() helper (batch collect + UDP send + BBR pacing)
• Rewrite stream_send(): channel send + oneshot await (no Mutex)
• Rewrite stream_recv(): channel send + oneshot await
• Rewrite open_stream(), stats(), close(): same pattern
• Make poll() a no-op
• Remove connection: Arc<Mutex<Option<QuicConnection>>> field
• Add send_tx: mpsc::Sender<SendCommand> field
• Spawn I/O driver in connect() after handshake completes

bins/quick-entry/src/pool.rs

• Remove start_background_tasks() manual poll loop (I/O driver replaces it)
• Store I/O driver handles for shutdown

bins/quick-entry/src/main.rs

• Remove manual poll loop (lines 772-805)
• Health checks via periodic GetStats commands

Acceptance Criteria

• QuicConnection owned exclusively by I/O driver (no shared Mutex)
• stream_send() uses bounded mpsc channel (capacity 64)
• Results returned via oneshot channels
• poll() is a no-op
• 351+ tests pass, CI green
• Proxmox benchmark: terrestrial upload >30 Mbps (currently 8.27)
• No memory leak under sustained load (bounded channel caps at 4MB)

Dependencies

Requires #110 (mutex split) merged — DONE.

Expected Impact

Upload: 8 Mbps → 50-80 Mbps terrestrial. GEO: 0 → 0.5+ Mbps.