create / write / truncate / remove / mkdir / rename, completing the
engine's pub-fn contract with the shared harness. The allocation BITMAP
is the authority: setAllocated IS the allocation (a set bit), and the
32-bit FAT only records a fragmented chain's order — forgetting the bit
would hand a live cluster out twice, so the write path never touches the
FAT without also owning the bit.
This engine writes FAT-linked (no_fat_chain=0) files: createFile makes
an empty set; writeFile allocates+links+zeroes clusters (so a sparse gap
reads zero and valid_data_length can honestly equal data_length) and
rewrites the Stream entry; a contiguous file opened for growth is first
threaded through the FAT. truncate frees the chain; removeFile clears
each set entry's InUse bit and frees the chain (a non-empty directory is
refused); rename re-homes the same clusters under a new name set.
createDirectory allocates one zeroed cluster — exFAT directories carry no
"." / ".." entries. Every entry-set mutation recomputes the set
checksum. Offsets clamp to the vfs u32 surface.
Ten engine host tests now (read + write across clusters, truncate+reuse,
remove, subdir+inner file, rename); 17 total with on-disk. bounds green.
Daniel Samson
e240341bfb