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/LVGL_Driver.cpp

@@ -1,4 +1,5 @@
 #include "LVGL_Driver.h"
+#include "SPI_Bus_Lock.h"
 
 static lv_disp_draw_buf_t draw_buf;
 static lv_color_t buf1[LVGL_BUF_LEN];
@@ -187,5 +188,6 @@ void Lvgl_Init(void) {
 }
 
 void Timer_Loop(void) {
+  if (SPIBus_IsDisplayPaused()) return;
   lv_timer_handler();
 }