fix(bench): escape the NUL separator so measure.mjs stays a text file

The fingerprint key separator was written as a literal NUL byte instead of the
`\u0000` escape. Git classifies any file containing a NUL as binary, so the
whole bench showed as `Bin` with no diff on GitHub and could not be reviewed —
the same defect this branch already fixed once before the tri-review.

Escaping it is byte-for-byte equivalent at runtime (both produce U+0000), so
the committed fingerprint is unchanged and `--check` still passes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

gitnexus/bench/cpp-qualified-ns/measure.mjs

@@ -397,7 +397,7 @@ function outcomesOf(parsedFiles, sites) {
       callsite,
     );
     outcomes.add(
-      `${siteKey(receiver, member, callsite)}${hit === undefined ? '<none>' : hit === 'ambiguous' ? '<ambiguous>' : hit.nodeId}`,
+      `${siteKey(receiver, member, callsite)}\u0000${hit === undefined ? '<none>' : hit === 'ambiguous' ? '<ambiguous>' : hit.nodeId}`,
     );
   }
   return outcomes;