Check downloaded originals against their recorded content hash (closes #68)

downloadFile, shared by quak get, the content cache and backup, hashes
the decrypted bytes (unkeyed BLAKE2b-512, standard base64) and stores
nothing on a mismatch, failing with an error naming the file ID. A live
photo ZIP is unpacked as it streams with fflate's Unzip, in small
slices so memory stays bounded however far an entry expands, and its
image and video hashed separately as <imageHash>:<videoHash>.
decryptFile reads the older imageHash/videoHash fields for live
photos. A file with no recorded hash is stored unchecked.

Model: opus-5-5

TODO.md

@@ -18,6 +18,14 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-23: Checked downloaded originals against their recorded content hash
+  (issue 68). `downloadFile`, which `quak get`, the content cache and backup all
+  use, hashes the decrypted bytes (unkeyed BLAKE2b-512, standard base64) and
+  stores nothing on a mismatch, failing with an error naming the file ID. A live
+  photo ZIP is unpacked as it streams with `fflate` and its image and video
+  hashed separately as `<imageHash>:<videoHash>`. `decryptFile` reads older
+  clients' `imageHash` and `videoHash` fields for live photos. A file with no
+  recorded hash is stored unchecked.
 - 2026-09-23: Kept one account's cache from mixing with another's (issue 104).
   When `metadata.json` in the cache directory was written for a different,
   non-zero user ID than the client's, `Library.open` deletes it and `mldata/`