feat(tunnel): fully separated upload/download tasks, wseq write ordering

Architecture:
- Upload task (spawned): reads client socket → sends MuxMsg::Data with
  wseq directly to mux → sends InflightEntry to download task. Fully
  independent, never blocked by downloads.
- Download task (inline): processes replies, sends refill polls (timer),
  accepts InflightEntry. Never blocked by uploads.
- Lock-free mpsc channels throughout — no Mutex contention.

Write ordering (wseq):
- Client assigns monotonic wseq to data-bearing ops only (not polls).
- Tunnel-node buffers out-of-order writes per session, flushes in wseq
  order. Backward compatible: old clients without wseq write immediately.
- Fixes data corruption from pipelined batches completing out of order.

Upload accumulation:
- Adaptive: 50ms initial window for small messages (low latency).
- If >= 32KB accumulated, extend to 1s / 1MB cap (fat uploads for files).

Other:
- Removed consecutive_empty gate on refill (was killing idle sessions).
- Tunnel-node reader buffer 2MB (was 512KB).
- Removed legacy detection (was false-triggering on merged replies).

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

Author: vahidlazio

src/domain_fronter.rs

@@ -532,6 +532,8 @@ pub struct BatchOp {
     pub d: Option<String>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub seq: Option<u64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub wseq: Option<u64>,
 }
 
 /// Batch tunnel response from Apps Script / tunnel node.