Add serial upload tool for rapid Lua game iteration

Implements a complete serial upload workflow that allows uploading and
immediately testing Lua games via USB serial connection.

New Components:
- SerialUploader: Receives files via serial, writes to SD card
- upload_game.py: Python tool for sending files from host computer
- Protocol: Text-based with base64 encoding for reliability

Key Features:
- Uploads file to /games folder on SD card
- Overwrites existing files (FA_CREATE_ALWAYS)
- Auto-launches uploaded game immediately
- Proper memory cleanup (prevents Lua state conflicts)

SD Card Fixes:
- Fixed SPI speed management (12.5MHz for SD, 32MHz for display)
- Fixed SD write protocol (poll for data response token)
- Added speed switching wrappers around all FatFS operations
- Cleaned up excessive debug output

Game Launcher Improvements:
- Added clear_games() to prevent duplicate registrations
- Added cleanup in select_game_by_name() to delete old instances
- Added exact match priority in game selection
- LuaGameLoader now has clear_factory_data() for memory cleanup

Integration:
- Added serial_uploader to CMakeLists.txt
- Integrated into main loop in basic1.cpp
- Re-scans games after upload to pick up new files

Documentation:
- UPLOAD_TOOL.md: Usage instructions
- sd_card_best_practices.md: Critical lessons learned

Known Issues:
- Game launch after upload occasionally causes freeze (needs investigation)
- Display may not refresh properly after upload

Usage:
  python upload_game.py games/lua_examples/2048.lua /dev/tty.usbmodem101

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

lib/serial_uploader.h

@@ -0,0 +1,52 @@
+#ifndef SERIAL_UPLOADER_H
+#define SERIAL_UPLOADER_H
+
+#include <cstdint>
+#include "ff.h"
+
+class GameLauncher;
+
+class SerialUploader {
+public:
+    SerialUploader(GameLauncher* launcher);
+
+    // Process incoming serial data (call this frequently in main loop)
+    // Returns true if a game was launched
+    bool process();
+
+    // Get the filename of the last uploaded game (without .lua extension)
+    const char* get_last_uploaded_filename() const { return last_uploaded_name; }
+
+private:
+    enum State {
+        IDLE,
+        RECEIVING_FILENAME,
+        RECEIVING_SIZE,
+        RECEIVING_DATA,
+        WRITING_FILE,
+        LAUNCHING_GAME
+    };
+
+    State state;
+    GameLauncher* game_launcher;
+
+    // Upload state
+    char filename[64];
+    char last_uploaded_name[64];  // Game name without .lua extension
+    uint32_t file_size;
+    uint32_t bytes_received;
+    uint8_t* file_buffer;
+
+    // Base64 decoding buffer
+    char base64_buffer[4];
+    int base64_index;
+
+    // Helper methods
+    void reset();
+    bool write_file_to_sd();
+    void launch_game();
+    uint8_t decode_base64_char(char c);
+    void decode_base64_block(const char* input, uint8_t* output);
+};
+
+#endif // SERIAL_UPLOADER_H