Pin the #276 headline behaviour RED-first: N threads sharing one
Registry handle and concurrently appending families must leave
families.jsonl well-formed — every member of every successful append
survives as its own complete, parseable JSON line (none torn, merged
into a neighbour, or lost). Today's unsynchronized OpenOptions::append
+ writeln! interleaves concurrent writers' content and newline writes,
so the test fails deterministically (verified 3/3 runs: "trailing
characters" merged-line parse error).
The sharing topology — one shared &Registry across scoped threads —
models the planned parallel campaign cell loop's caller shape and
admits any of the issue's candidate fixes.
refs #276
claude
494b7885c7