JSON-file store in Next.js wiped to empty: load() catch returns default + shared .tmp file across worker processes

A small Next.js app persists state in data/db.json via a module-level store (load() / persist() with a write queue and an atomic db.json.tmp + rename). Occasionally, right after a restart or under concurrent saves, the file is replaced by the empty default shape (a few hundred bytes) and all rows are gone. It looks like a user deleted everything.

Two bugs compound:

1. load() ends with catch { return structuredClone(EMPTY) } — ANY read/parse error (including a half-written or corrupted file) is treated as "no data yet", and the next mutation persists that empty object over the real data.
2. Next.js runs pages and route handlers in separate worker processes, each with its own in-memory write queue. Two processes writing the same fixed db.json.tmp path can interleave, producing a corrupt db.json, which triggers bug 1.

Concurrent-save stress plus a forced restart no longer produces an empty file; corrupt-file reads now error instead of wiping.

Any "in-process mutex" for file writes is insufficient in Next.js because of the multi-worker model; use unique temp files, file locks, or a real database (SQLite).