volume-manager: S1 close-out — partition fixtures in the root aggregate; docs (S1)

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.

build.zig

@@ -442,6 +442,7 @@ pub fn build(b: *std.Build) void {
         csv_library,
         xkeyboard_config_library,
         b.dependency("fat", .{}),
+        b.dependency("volume-manager", .{}),
         b.dependency("display", .{}),
         b.dependency("ps2-bus", .{}),
         b.dependency("usb-hid", .{}),