Add monitord, a rotating CSV logger

[evandhoffman]

Closes #41.

Adds monitord, a headless daemon that samples every metric on the same clock and writes rotating, human-readable CSV — the background-sampler half of the persistence roadmap, aimed at a different consumer than the app's own history: other processes that want to correlate performance with temperature or throttling.

• MonitorCore: LogRetention (window, cadence, period) and CSVLogFormat (wide CSV, reusing the CSVExport helpers). Pure and testable.
• MonitorLog: CSVLogSink, an actor that writes one file per cadence period (hourly for sub-day retention, daily otherwise), rolls on the boundary, and sweeps old files on a slow timer.
• monitord: runs the existing Sampler with the sink; --dir, --retention, --interval; clean shutdown on SIGINT/SIGTERM.

The app still has no code path that reaches the filesystem: monitor does not link MonitorLog.

Format

• Wide: one row per timestamp, one column per metric. Header written once per file from the discovered metrics.
• Two time columns: ISO8601 (UTC) and epoch millis.
• A metric that misses a tick leaves an empty field, never a zero.
• Hostname is a column and part of the filename, so files from several machines sharing a directory do not clobber.

Rotation

• One file per day, boundary on 00:00:00 localtime; hourly for sub-day windows (1h, 6h), because a daily file cannot honor a sub-day window.
• Retention window: 1h, 6h, 24h, 48h, 3d, 5d, 7d, 14d, 30d, forever.

Verification

• swift build && swift test — 227 tests pass (6 new).
• swiftformat --lint — clean.
• swift build -c release — clean.
• Smoke-tested monitord against a temp dir: correct hourly file, hostname, timestamps, wide rows.