A C0 control character in a node label or id (from a mangled extraction or an odd
filename) crashed the GraphML export (XML 1.0 forbids C0 except tab/LF/CR) and the
Obsidian export (EINVAL on Windows paths), aborting the whole artifact. Fold a
control-char scrub into the existing per-format coercion hooks so only the illegal
characters are stripped; tab/LF/CR and non-ASCII letters are preserved, and to_json
(and its byte-identity round-trip) is untouched.
