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.cpp

@@ -153,7 +153,10 @@ void SerialUploader::launch_game() {
     // Note: LuaGameLoader::register_all_games handles SPI speed internally
     int new_games = LuaGameLoader::register_all_games(game_launcher);
     printf("Found %d Lua games after re-scan\n", new_games);
+}
 
+bool SerialUploader::complete_launch() {
+    // This should only be called when it's safe (no display refresh in progress)
     // Now try to launch the newly uploaded game by name
     printf("Attempting to launch game: %s\n", last_uploaded_name);
     bool launched = game_launcher->select_game_by_name(last_uploaded_name);
@@ -163,6 +166,11 @@ void SerialUploader::launch_game() {
     } else {
         printf("ERROR Failed to launch game: %s\n", last_uploaded_name);
     }
+
+    // Reset state back to IDLE
+    reset();
+
+    return launched;
 }
 
 bool SerialUploader::process() {
@@ -305,6 +313,8 @@ bool SerialUploader::process() {
     if (state == WRITING_FILE) {
         if (write_file_to_sd()) {
             state = LAUNCHING_GAME;
+            // Prepare for launch by scanning games, but don't actually launch yet
+            launch_game();
         } else {
             reset();
         }
@@ -312,9 +322,8 @@ bool SerialUploader::process() {
     }
 
     if (state == LAUNCHING_GAME) {
-        launch_game();
-        reset();
-        return true; // Signal that game was launched
+        // Stay in this state until main loop calls complete_launch() when safe
+        return false;
     }
 
     return false;