sleep --derive: hr_avg emitted at full float64 precision; total_sleep_min equals light_min

[DTTerastar]

Severity

minor

Summary

Two related cosmetic / data-hygiene problems in the --derive polyfill path of the sleep subcommand.

1. hr_avg precision blows up on derived rows

Real Withings rows in CSV print hr_avg as a small integer-ish value. Derived rows print 16 digits of float noise:

$ withings-export sleep --since 7d --derive 2>/dev/null | head -3
date,start,end,timezone,total_sleep_min,...,hr_avg,hr_min,hr_max,...,source
2026-04-18,...,Local,385,...,45.86206896551724,0,0,...,derived
2026-04-19,...,Local,567,...,43.037974683544306,0,0,...,derived

This is the same column non-derived rows fill with a single integer. Format with %.0f (matches markdown's fmtRound) or %.1f for parity with the fmtFloat1 helper already in cmd/shared.go.

2. Derived rows put all minutes into light_min

Every derived row has total_sleep_min == light_min and deep_min == rem_min == 0. That isn't surprising for an HR-only polyfill, but consumers can't tell whether "deep_min=0" means "this user had zero deep sleep" or "we don't actually know". For derived rows, please leave the per-stage columns blank/null rather than zero, or at minimum document this in prime / --help. Same applies to wakeup_count, time_to_sleep_sec, etc.

Reproduce

See command above.

Expected

• Round derived hr_avg to integer or one decimal in CSV/markdown.
• For derived rows, emit empty CSV cells / JSON null for fields the heuristic cannot infer (deep, rem, wakeups, latencies).

[DTTerastar]

Confirmed still reproduces against HEAD b580683. CSV: hr_avg=45.86206896551724 on derived rows; total_sleep_min == light_min and deep/rem/wakeup_count/latency all 0 rather than blank/null. JSON for derived rows leaks the same precision ("hr_average": 45.86206896551724). Markdown rounds to one decimal (HR avg 45.9) so the formatter helpers exist; the CSV/JSON paths just don't use them.

[Terastar-Paperclip]

Thanks — two tightly related --derive hygiene issues bundled together is fine here; both attach to the same code path.

Triage: kind:bug · area:format · priority:medium per TRIAGE.md. Reproducible serializer-side issue with a clear fix shape (apply existing fmtRound/fmtFloat1 helpers; emit nulls where the heuristic cannot infer). Related to #26 — same family (full float64 precision leaking through structured outputs) on a different subcommand.

Routing: Lead Go Engineer.