Backend selector for Noodle: SdFat, SD_MMC, FFat, LittleFS, or NONE. Noodle supports multiple storage backends (SdFat, SD_MMC, FFat, LittleFS, or NONE). Define exactly ONE of:

NOODLE_USE_SDFAT
NOODLE_USE_SD_MMC
NOODLE_USE_FFAT
NOODLE_USE_LITTLEFS
NOODLE_USE_NONE // <— no filesystem / no storage

Exposes:

NDL_File (FsFile for SdFat, File otherwise, or NDL_NullFile for NONE)
NOODLE_FS (SdFat object for SdFat; FFat/SD_MMC/LittleFS singleton otherwise; not used for NONE)
noodle_fs_open_read / noodle_fs_open_write / noodle_fs_remove
other file operations

Path normalization: Some Arduino filesystem APIs require paths to start with a leading slash (e.g. "/w01.txt"), while SdFat commonly uses bare names (e.g. "w01.txt"). The helper noodle_norm_filename() applies this rule.

Warning: noodle_norm_filename() returns a pointer to a static buffer when a leading slash is needed. That means it is not re-entrant and not thread-safe: do not store the returned pointer; use it immediately for open/remove.