Add volume-manager to build.zig's root package-test loop, so `zig build test`
runs the partition parser's nine host tests and a fixture added there can never
be silently skipped. Flip the identity-ladder build status in
storage-design-rationale.md: rungs 1 (GPT partition GUID) and 3 (FAT serial +
label) are built, joining rung 4; rung 2 (filesystem UUID) waits on a non-FAT
engine; the volumes.csv map and the id-derived mount path land with S2.
Daniel Samson
aca3d5855a