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.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <Arduino.h>
+
+void SPIBus_Init();
+void SPIBus_Lock();
+void SPIBus_Unlock();
+void SPIBus_SetDisplayPaused(bool paused);
+bool SPIBus_IsDisplayPaused();