esp32/boards: Raise default lwIP TCP window on PSRAM boards. - #19592

Raises the default lwIP TCP window on esp32 boards with external PSRAM,
which currently run with a window too small to use the WiFi link.

The problem

The esp32 port uses the lwIP default TCP send buffer / window of 5760
bytes. A single TCP connection is then limited to window / RTT; at a
typical WiFi RTT of ~5 ms that caps throughput at roughly 5-10 Mbit/s
regardless of the WiFi PHY rate or band -- the extra capacity of
5 GHz / 802.11ax is wasted. The ESP32-P4 board already raises this in its
own config; other boards don't.

The fix

Set CONFIG_LWIP_TCP_SND_BUF_DEFAULT and CONFIG_LWIP_TCP_WND_DEFAULT to
65534 in sdkconfig.spiram_oct and sdkconfig.spiram_quad, covering the
S2/S3/C5 boards that use quad/octal PSRAM. It is gated on PSRAM
deliberately: the larger per-connection buffers cost internal RAM, so
non-PSRAM boards keep the small window (size over speed). The
original-ESP32 PSRAM config (sdkconfig.spiram_esp32) is left unchanged
for now -- those chips have a lower WiFi PHY and less RAM headroom and I
have not tested them; it can be a follow-up.

Testing

Raw single-connection TCP TX throughput (RSSI -40..-48 dBm). The ESP32-C5
was on an 802.11ax (WiFi 6) AP; the S3 figure is a generic ESP32-S3
module:

The cap was confirmed window-limited (effective in-flight data ~= 5760 B,
the default), and opening several parallel connections scaled aggregate
throughput before the fix -- i.e. the WiFi PHY had headroom a single
connection could not use.

Generative AI

I used generative AI tools when creating this PR, but a human has checked the
code and is responsible for the code and the description above.