apk/installed: parse Z: (file checksum) token in ParseInstalled

[Dev-X25874]

AddInstalledPackage writes a Z:<checksum> line for every regular
file that carries a checksum, and real installed databases are full of
them. ParseInstalled had no case "Z": handler, so those lines were
silently dropped on read-back — every reconstructed tar.Header came
back with a nil PAXRecords map and no checksum.

Add the missing case, mirroring the write path: allocate PAXRecords
on demand and store the value under paxRecordsChecksumKey, the same
key used by installRegularFile and AddInstalledPackage.

[Dev-X25874]

Hey @a-crate — would you mind taking a look at this when you get a chance? This adds the missing Z: case in ParseInstalled to handle file checksums that were being silently dropped on read-back. Happy to answer any questions or make changes based on your feedback!