Remove the post-fetch regex sanitization sweep — the in-fetcher
sanitizer is the single source of truth. Addresses review comment
#3183044004 (in part).
Walks pack and node fields explicitly with the user-content extension
regex instead of regex-scanning the entire JSON blob (the prior anchor
$ only matched the very end of the serialized string). Validation now
runs before the temp file is renamed into place so a leak aborts the
refresh without ever overwriting the committed snapshot. Addresses
Oracle review finding #2.
