handshake-capture-c6: SPI bus lock, SD v1.0.2 diagnostics, MUST_DO review

- SPI_Bus_Lock: serialize LCD + SD on shared SPI; pause display during SD
- SD_Init after WiFi scan; SD_IsReady gating; serial [SD]/[CAP] diagnostics
- MUST_DO_IMPROVEMENTS.md full firmware review backlog; README link
- Bump HANDSHAKE_FIRMWARE_VERSION to 1.0.2 (header)

Made-with: Cursor

handshake-capture-c6/SPI_Bus_Lock.cpp

@@ -0,0 +1,37 @@
+#include "SPI_Bus_Lock.h"
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+
+static SemaphoreHandle_t s_spi_bus_mutex = nullptr;
+static volatile bool s_display_paused = false;
+
+static void ensureMutex() {
+  if (s_spi_bus_mutex == nullptr) {
+    s_spi_bus_mutex = xSemaphoreCreateRecursiveMutex();
+  }
+}
+
+void SPIBus_Init() {
+  ensureMutex();
+}
+
+void SPIBus_Lock() {
+  ensureMutex();
+  if (s_spi_bus_mutex != nullptr) {
+    xSemaphoreTakeRecursive(s_spi_bus_mutex, portMAX_DELAY);
+  }
+}
+
+void SPIBus_Unlock() {
+  if (s_spi_bus_mutex != nullptr) {
+    xSemaphoreGiveRecursive(s_spi_bus_mutex);
+  }
+}
+
+void SPIBus_SetDisplayPaused(bool paused) {
+  s_display_paused = paused;
+}
+
+bool SPIBus_IsDisplayPaused() {
+  return s_display_paused;
+}