Support littlefs as the CIRCUITPY filesystem

[tannewt]

Add a supervisor-level filesystem API (supervisor_vfs_*) that works on both FAT and littlefs mounts so workflows, BLE file transfer, the web workflow and settings don't care which filesystem is active. The root filesystem storage becomes a supervisor_vfs_t union of fs_user_mount_t (FAT) and mp_obj_vfs_lfs2_t (littlefs); the root is mounted as littlefs when CIRCUITPY_FILESYSTEM_LITTLEFS is set.

• extmod: export mp_obj_vfs_lfs2_t and add mp_vfs_lfs2_mount_supervisor() so the supervisor can mount littlefs before the VM and GC run, and make the littlefs VFS honor the supervisor write-protection flags so storage.remount() applies to it like FAT.
• supervisor: map FatFS and littlefs errors to shared supervisor_fs_err_t codes, carry mtimes as ns past 1970 and stamp littlefs mtime attributes and FAT timestamps from the same value.
• workflow/USB/BLE/web code switch from FRESULT to supervisor_fs_err_t and use the new API, removing direct FatFS access.
• zephyr-cp: pick FAT or littlefs per board from the partition layout (a littlefs_partition devicetree node selects littlefs), add a native_sim_lfs test harness, fix the flash write cache stale-read when the target block is in the currently cached page, and provide mp_hal_time_ns().