Add sensor logging

[evandhoffman]

We're already monitoring power, temperature, and fan sensors. It would be nice if these data could be logged to a file and then used by other processes to correlate performance with temperature/throttling.

Ideas:

• logging interval 1s, 2s, 5s, 10s
• Maximum file size - circular files?
• Maximum run time? Don't want to forget about something that's just logging infinitely
• file rotation?
• Just log to syslog? Avoids a bunch of file management issues.

[evandhoffman]

Decision: rotating CSV log

After working through the options, the logger is a rotating, human-readable CSV written by a background daemon (monitord) — not SQLite, not syslog.

Why not SQLite: not human-readable, and the consumer here is "open the file and read it" plus "feed it to another process." Why not syslog: it is a text stream with OS-managed retention; correlation needs the data in a form a tool loads, and CSV is that form.

Format

• Wide: one row per timestamp, one column per metric. The header is written once per file from the discovered metrics, so a fanless Mac has no fan column.
• Two time columns: ISO8601 (with explicit offset) and epoch millis.
• A sensor that misses a tick leaves an empty field, never a zero — same rule the app uses, so a gap does not read as a cold die.
• Hostname is a column and part of the filename, so files from multiple machines sharing a collector do not clobber.

Rotation

• One file per day, boundary on 00:00:00 localtime.
• Sub-day windows (1h, 6h) use hourly files, because daily files cannot honor a sub-day window — a "1h" window would keep up to 24h of data.
• Retention window, user-selectable: 1h, 6h, 24h, 48h, 3d, 5d, 7d, 14d, 30d, forever. "Forever" means you own the disk.

Daemon

• monitord runs the existing Sampler (one clock, honors the sensor 1 Hz minimum interval) with a new CSV sink. The app ring buffer is untouched.

[evandhoffman]

Decided spec (implemented in #42):

Format — rotating, human-readable CSV, one file per day. Wide: one row per timestamp, one column per metric. Header written once per file.

File naming — sensors.<hostname>.<date>.log, e.g. sensors.macbook-pro.local.2026_09_04.log. The day is baked into the name, so files from several machines sharing a directory do not clobber. At most one file per day.

Columns — hostname, time_iso8601 (UTC), time_epoch_ms, then one column per metric. Temperatures appear twice, in °C and °F. A metric that produced no sample that tick leaves an empty field, never a zero.

Precision — fractional values to 2 decimals; RPM and byte counts are whole numbers.

Retention — user-selectable window: 1h, 6h, 24h, 48h, 3d, 5d, 7d, 14d, 30d, forever. Deletes whole files older than the window. Because files are daily, a sub-day window keeps today's file (up to a day of data) — the window is a floor, not a promise at sub-day granularity.

Sampling — same clock as the app, honors each source's minimum interval. Run as a launchd LaunchAgent to log for days.