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>

spi_gpio_probe/CMakeLists.txt

@@ -0,0 +1,3 @@
+cmake_minimum_required(VERSION 3.16)
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(spi_gpio_probe)

spi_gpio_probe/main/CMakeLists.txt

@@ -0,0 +1 @@
+idf_component_register(SRCS "main.c" INCLUDE_DIRS ".")

spi_gpio_probe/main/main.c

@@ -0,0 +1,103 @@
+// Standalone diagnostic firmware - NOT part of the real esp-spi project.
+// Purpose: bypass the SPI slave peripheral entirely and watch the raw GPIO
+// pins (plain digital input, interrupt on every edge) that our real
+// firmware's spi_slave.cpp configures for SPI slave use on ESP32-S3:
+//   CS=GPIO10, SCLK=GPIO12, MOSI=GPIO13, MISO=GPIO11
+// Logs every CS/SCLK edge with a microsecond timestamp and the
+// MOSI/MISO/CS level sampled at that instant, so a real transaction
+// attempt from the CH341A bridge can be visually reconstructed from the
+// console log - answers directly whether the ESP32 sees anything at all
+// on these pins, and if so, whether it looks like a sane SPI waveform
+// (CS low, some number of SCLK edges, CS high) or not.
+#include <stdio.h>
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+#include "freertos/queue.h"
+#include "driver/gpio.h"
+#include "esp_timer.h"
+
+#define PIN_CS   10
+#define PIN_SCLK 12
+#define PIN_MOSI 13
+#define PIN_MISO 11
+
+typedef struct
+{
+    int64_t us;
+    uint8_t pin;    // which pin's edge triggered this event (PIN_CS or PIN_SCLK)
+    uint8_t level;  // level that pin transitioned TO
+    uint8_t cs;
+    uint8_t sclk;
+    uint8_t mosi;
+    uint8_t miso;
+} event_t;
+
+#define EVQ_LEN 4096
+static QueueHandle_t evq;
+
+static void IRAM_ATTR gpio_isr(void *arg)
+{
+    uint32_t pin = (uint32_t)(uintptr_t)arg;
+    event_t ev;
+    ev.us    = esp_timer_get_time();
+    ev.pin   = (uint8_t)pin;
+    ev.level = (uint8_t)gpio_get_level((gpio_num_t)pin);
+    ev.cs    = (uint8_t)gpio_get_level(PIN_CS);
+    ev.sclk  = (uint8_t)gpio_get_level(PIN_SCLK);
+    ev.mosi  = (uint8_t)gpio_get_level(PIN_MOSI);
+    ev.miso  = (uint8_t)gpio_get_level(PIN_MISO);
+
+    BaseType_t hpw = pdFALSE;
+    xQueueSendFromISR(evq, &ev, &hpw);
+    if (hpw) portYIELD_FROM_ISR();
+}
+
+void app_main(void)
+{
+    printf("\n\n=== spi_gpio_probe starting ===\n");
+    printf("Pins: CS=GPIO%d SCLK=GPIO%d MOSI=GPIO%d MISO=GPIO%d\n", PIN_CS, PIN_SCLK, PIN_MOSI, PIN_MISO);
+    printf("Watching CS + SCLK edges only (plain digital input, no SPI peripheral involved).\n\n");
+
+    evq = xQueueCreate(EVQ_LEN, sizeof(event_t));
+
+    gpio_config_t io = { 0 };
+    io.pin_bit_mask = (1ULL << PIN_CS) | (1ULL << PIN_SCLK) | (1ULL << PIN_MOSI) | (1ULL << PIN_MISO);
+    io.mode = GPIO_MODE_INPUT;
+    io.pull_up_en = GPIO_PULLUP_DISABLE;
+    io.pull_down_en = GPIO_PULLDOWN_DISABLE;
+    io.intr_type = GPIO_INTR_ANYEDGE;
+    gpio_config(&io);
+
+    gpio_install_isr_service(0);
+    gpio_isr_handler_add((gpio_num_t)PIN_CS,   gpio_isr, (void*)(uintptr_t)PIN_CS);
+    gpio_isr_handler_add((gpio_num_t)PIN_SCLK, gpio_isr, (void*)(uintptr_t)PIN_SCLK);
+
+    printf("Idle levels right now: CS=%d SCLK=%d MOSI=%d MISO=%d\n\n",
+           gpio_get_level(PIN_CS), gpio_get_level(PIN_SCLK),
+           gpio_get_level(PIN_MOSI), gpio_get_level(PIN_MISO));
+
+    int64_t last_us = 0;
+    unsigned long total_events = 0;
+    unsigned long cs_high_count = 0;
+
+    for (;;)
+    {
+        event_t ev;
+        if (xQueueReceive(evq, &ev, pdMS_TO_TICKS(3000)))
+        {
+            const char *pin_name = (ev.pin == PIN_CS) ? "CS  " : "SCLK";
+            long long delta = last_us ? (long long)(ev.us - last_us) : 0;
+            printf("[t=%10lld us  d=%6lld us] %s -> %d   | CS=%d SCLK=%d MOSI=%d MISO=%d\n",
+                   (long long)ev.us, delta, pin_name, ev.level,
+                   ev.cs, ev.sclk, ev.mosi, ev.miso);
+            last_us = ev.us;
+            total_events++;
+            if (ev.pin == PIN_CS && ev.level == 1) cs_high_count++;
+        }
+        else
+        {
+            printf("--- idle (no CS/SCLK edges in last 3s) --- total_events=%lu cs_high_transitions=%lu\n",
+                   total_events, cs_high_count);
+        }
+    }
+}