Initial import: ESP32 ZiFi32 firmware with SMB2/3 VFS bridge

Based on https://github.com/tslabs/zx-evo/tree/master/pentevo/esp32,
extended with an SMB2/3 server that exposes the ZX Evolution's real SD
card as a network share (browse/download/upload/delete), plus assorted
stability and protocol-correctness fixes in vfs_bridge.cpp and the
vendored libsmb2 - see firmware/README.md for the detailed changelog
and current status.

Also adds:
- firmware/: ready-to-flash compiled binaries + flashing instructions
- hardware/: adapter board connection diagram, PCB photo, and Gerber
  files
- src/third_party/libsmb2: vendored from sahlberg/libsmb2 (LGPL-2.1,
  see its own COPYING/LICENCE files), de-submoduled to a plain
  snapshot with local patches applied directly

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

test/rpi/readme.txt

@@ -0,0 +1,33 @@
+
+--- 1. Enable SPI
+sudo nano /boot/firmware/config.txt
+
+Add or uncomment:
+dtparam=spi=on
+
+sudo reboot
+
+ls -l /dev/spidev*
+
+
+--- 2. Connect ESP-SPI
+
+For ESP32-S3:
+
+MOSI    GPIO13
+MISO    GPIO11
+SCLK    GPIO12
+CS      GPIO10
+
+For ESP32-P4:
+MOSI    GPIO20
+MISO    GPIO22
+SCLK    GPIO21
+CS      GPIO23
+
+RPi:
+MOSI = GPIO10 (pin 19)
+MISO = GPIO9  (pin 21)
+SCLK = GPIO11 (pin 23)
+CS0  = GPIO8  (pin 24)
+CS1  = GPIO7  (pin 26)