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>

.gitignore

@@ -0,0 +1,14 @@
+# ESP-IDF build output (regenerate with `idf.py build`)
+build/
+.test-build/
+managed_components/
+sdkconfig.old
+
+# Editor/IDE cruft
+.vscode/
+.idea/
+*.user
+
+# OS cruft
+Thumbs.db
+.DS_Store