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/SD_Card.h

@@ -15,3 +15,4 @@ bool SD_Init();
  * with LCD+SD sharing SPI, cardType() can spuriously read CARD_NONE and falsely show "insert SD".
  */
 bool SD_IsReady();
+bool SD_WriteFileAtomic(const char* path, const uint8_t* data, size_t len);