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>

src/main/vfs_bridge.h

@@ -0,0 +1,21 @@
+#pragma once
+
+// Phase 1 of the SMB2/3 network-share feature (see project plan): an SMB2/3
+// server task with a hardcoded fake filesystem, to prove the protocol layer
+// works before anything is wired to the Z80/Wild Commander side.
+
+#define VFS_SHARE_NAME_MAX 32
+extern char vfs_share_name[VFS_SHARE_NAME_MAX + 1];
+
+void vfs_bridge_init();
+void smb_server_task(void *arg);
+
+// Start/stop the SMB2/3 server on demand (ESP_CMD_VFS_SERVER_START/STOP,
+// dispatched via TASK_VFS_SERVER_START/STOP in helper_task) instead of it
+// running unconditionally once WiFi comes up - the Z80 side is single-
+// tasking (it can only ever be driving one plugin at a time), so there is
+// no point keeping the SMB server (or the other services it pauses -
+// at_uart_suspend()/tracker_suspend()) active except while the smbvfs.wmf
+// plugin is actually the thing running.
+void vfs_bridge_start_server();
+void vfs_bridge_stop_server();