Fix SPI bus contention crash on serial game upload

Serial uploader was crashing the Pico when launching games because
it accessed SD card (SPI) while Core 1 was refreshing display (also SPI).
Display and SD card share the same SPI bus and cannot be accessed
simultaneously.

Split game launch into prepare and execute phases:
- prepare: Re-scan games directory (safe, SD access done immediately)
- execute: Load Lua script from SD (deferred until display is idle)

Main loop now checks !is_refresh_in_progress() before completing
launch, preventing SPI conflicts.

Also updated SD card best practices skill to document SPI bus
contention as the #1 most critical issue to avoid.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/serial_uploader.h

@@ -14,6 +14,13 @@ public:
     // Returns true if a game was launched
     bool process();
 
+    // Check if uploader wants to launch a game (after upload complete)
+    bool wants_to_launch_game() const { return state == LAUNCHING_GAME; }
+
+    // Complete the game launch (call only when safe - no display refresh in progress)
+    // Returns true if launch succeeded
+    bool complete_launch();
+
     // Get the filename of the last uploaded game (without .lua extension)
     const char* get_last_uploaded_filename() const { return last_uploaded_name; }