lwip: raise TCP_WND 5760->23040, RECVMBOX 6->32 (window step 1)

Instrumented baseline at WND=5760 (kitchen, ~202KB frames, 20fps
sender) confirmed the receive window as the dominant throttle:

- wire 53.0/58.8 Mbps avg/max = the WND/RTT ceiling, vs ~94 Mbps
  line rate on the 10/100 PHY
- recv_chunk_max pinned at exactly 5760 (drains window-quantized)
- recv_calls avg 60/frame; queued_at_body always 0
- sender backpressured 44-61% of frames, drop-oldest 17-54/window
- meanwhile pend_age avg 58us: decode idles, receive starves it

Expected: recv_avg ~30ms -> ~20ms, wire toward line rate, sender
bp%/drop-oldest down, g2g 73ms -> ~60ms. Tripwires (revert if hit):
pend_age or recv_dropped_oldest growing window-over-window, g2g
regressing. RECVMBOX scales with the window: 23040 = 16 segments
in flight, a 6-deep mbox would silently become the new cap.

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