ACCEPTANCE CRITERIA (14/14)

☑ 1. `lib/data-store.js` exports `readJSON(filePath)`, `writeJSON(filePath, data, options)`, `appendToSeries(filePath, dataPoint, maxEntries)`, `ensureDir(dirPath)`

☑ 2. `readJSON` returns parsed object or `null` if file does not exist or is invalid JSON

☑ 3. `writeJSON` writes atomically: write to `.tmp` file then `fs.renameSync` to final path

☑ 4. `writeJSON` creates parent directories automatically via `ensureDir`

☑ 5. `writeJSON` supports `options.backup` (creates `.bak` before overwrite) and `options.pretty` (2-space indent, default true)

☑ 6. `appendToSeries` deduplicates by `date_iso` field -- same date overwrites existing entry

☑ 7. `appendToSeries` trims series to `maxEntries` (default 365) oldest-first

☑ 8. `appendToSeries` returns the updated series object with `lastUpdated` timestamp

☑ 9. `lib/logger.js` exports `createLogger(name, options)`

☑ 10. Logger returns object with `.debug()`, `.info()`, `.warn()`, `.error()` methods

☑ 11. Output format: `TIMESTAMP [LEVEL] name -- message {JSON_DATA}`

☑ 12. Level filtering: messages below configured level are suppressed (debug < info < warn < error)

☑ 13. Optional file output: if `options.file` is set, also appends to that file path

☑ 14. Both libs use ONLY Node.js stdlib (`fs`, `path`) -- zero npm dependencies