Add update-only mode for test dataset generator

Add support for running content updates against an existing migration
test dataset without recreating the filesystem structure.

Also make ACL/xattr updates non-fatal on filesystems that do not
support those operations.

cdssync/AGENTS.md

@@ -19,6 +19,9 @@ For migration test datasets in this workspace, follow this process by default:
   - omit it to create the dataset once and exit
   - use `0` for continuous random content updates
   - use any integer greater than `0` to rewrite mutable files every `N` seconds
+- The generator script also accepts `--update-only`:
+  - use it to update an existing dataset in place without recreating files, links, or directories
+  - combine it with `UPDATE_INTERVAL_SECONDS` to keep mutating an existing dataset on a fixed interval
 - If ACL/xattr coverage matters, ensure the generation host has:
   - `acl` installed for `setfacl` and `getfacl`
   - `attr` installed for `setfattr` and `getfattr`
@@ -55,3 +58,4 @@ Preferred verification commands include:
 
 - If the destination host lacks `acl` or `attr`, ACL/xattr verification will be incomplete.
 - If the destination filesystem does not support ACLs or xattrs, those attributes may not survive transfer even when the copy method is correct.
+- The generator now logs and continues when ACL/xattr assignment is unsupported on the target filesystem instead of exiting.