fatfs-integration

# FatFs Integration ## Overview Use this skill to integrate FatFs by proving the disk I/O layer before trusting filesystem behavior. FatFs is portable C, but the target-specific `diskio` functions, sector geometry, mount policy, and media timing decide whether it works reliably. ## When To Use Use this skill when: - The user wants to add or debug FatFs on SD card, eMMC, USB MSC, RAM disk, SPI flash, or block devices. - The issue involves `f_mount`, `f_open`, `f_read`, `f_write`, `FR_DISK_ERR`, `FR_NOT_READY`, `FR_NO_FILESYSTEM`, long filenames, Unicode, or exFAT. - The project uses `ff.c`, `ffconf.h`, `diskio.c`, or `FATFS`. Do not use this skill when the raw block device cannot read/write sectors yet; debug the storage driver first. ## First Questions Ask for: - Media type and transport: SDIO, SPI SD, eMMC, USB MSC, RAM disk, or flash translation layer. - Sector size, sector count, erase/block size, and whether writes are cached. - `ffconf.h`, `diskio.c`, and current FatFs return code. - RTOS or bare-metal runtime and whether reentrancy is enabled. - Whether formatting is allowed. Formatting can destroy user data. - Long filename, Unicode, exFAT, timestamp, and multi-volume requirements. ## Integration Checklist 1. Prove disk I/O. `disk_initialize`, `disk_status`, `disk_read`, `disk_write`, and `disk_ioctl` must work independently. 1. Match sector geometry. FatFs expects logical sectors and correct sector count. `GET_SECTOR_COUNT`, `GET_SECTOR_SIZE`, and `C...