I don’t use APFS on the big spinning drives where the pix are. But they definitely are on my iMacs and MacBook ssd’s. Yea Lightroom has a Validate dng feature... of course it locks you into Adobe.
How do you think I could discriminate between file corruption and the user making an edit to a file?
(Perhaps if the mtime and filesize doesn't change, but the SHA does?)
The location for metadata depends on the application. For DNG workflows, the metadata is a separate location in the DNG file, with separate metadata and data checksums. For other workflows, the metadata is in a sidecar (a per image file), or stored in a database managed by the application.