The pure, host-testable byte layer of the second engine: the Main Boot
Sector (VBR) and the six 32-byte directory-entry types — Allocation
Bitmap, Up-case Table, Volume Label, File, Stream Extension, File Name —
as align(1) extern structs, plus the three exFAT checksums (boot region,
up-case table, directory-entry set), the name hash, and the packed
timestamp <-> Unix-epoch conversion.
geometryOf accepts only "EXFAT " + 0xAA55 + an all-zero MustBeZero
region; that last guard is the mutual exclusion with FAT — a FAT prober
reads a zero bytes-per-sector there and rejects the volume, and this one
rejects a FAT boot sector for want of the exFAT name. Wire-format widths
are named consts (spec facts, no bare literals) so the bounds gate stays
green; the layout is pinned by @offsetOf/@sizeOf tests.
Wired into the host-test aggregate directly for now; it moves into the
exfat package's own test step when that lands (step 4). 7/7 host tests,
bounds green.
Daniel Samson
56bd2e7678