chore: replace raw NUL separators with \0 escape in rows.ts / dataflow.ts

[rahlk]

src/build/neo4j/rows.ts and src/schema/v2/dataflow.ts use a literal NUL control byte as a template-literal key separator, so git renders both as binary diffs. That made contract-adjacent edits in the #26 cluster (the revived deferred edgeToSymbol gate; the L3 dedup-key mapping) land unreviewable via normal diff. Replace each raw NUL with the \0 escape — byte-identical output, restores text diffs.

Pre-existing; surfaced by the final review of epic #26.