Zoneless repeating event: occurrences are an hour off after DST, and a title-only --occurrence edit moves the day #590

Summary

I was working on a project to sync Hey Calendar to a Google Calendar (https://github.com/edooley/hey-gcal) and stumbled across this: for a repeating event saved without a time zone (created in HEY's web app, so starts_at_time_zone is null), occurrences after a daylight saving change are an hour off in two ways:

1. Read: the calendar period API behind hey event week / hey event day reports them at the series' original UTC time, while HEY's web calendar shows the correct local time.
2. Write (data change): hey event edit <series> --occurrence <id> --apply-to current with only --title saves that UTC-based time, so the meeting moves an hour in HEY's web calendar.

Steps to reproduce

1. Use a HEY account and machine in America/Chicago.
2. In HEY's web app, create a weekly event "Test" on Wednesday 2026-10-21, 9:00–10:00 AM.
3. Run hey event week 2026-11-04 --json. DST ends on 2026-11-01.
4. Open HEY's web calendar on 2026-11-04.
5. Edit only that day's title:

   hey event edit <series> --occurrence <series>_2026-11-04 --apply-to current --title "Test (title-only edit)"

6. Run hey event week 2026-11-04 --json again, and check the web calendar.

Expected

• Steps 3–4: the API and the web calendar agree: 9:00 AM CST, starts_at = 2026-11-04T15:00:00Z.
• Step 5: only the title changes. The day stays at 9:00 AM.

Actual

• 2026-10-21, before the DST change, is correct everywhere (14:00:00Z = 9:00 AM CDT).
• starts_at_time_zone is null on the series, its parent, every occurrence, and the written-out day.
• Zoned events (e.g. invites carrying America/New_York) are not affected.
• Passing the time explicitly (--start-time 09:00 --end-time 10:00) writes 15:00:00Z and fixes the day.

Where it comes from

• Read: hey-sdk's CalendarPeriods().Week / .Day return HEY's JSON unchanged, and hey event week doesn't modify starts_at. So the UTC-based time comes from HEY's period API, which disagrees with HEY's web calendar for the same day.
• Write: virtualOccurrence → occurrenceInstants (internal/cmd/events_occurrence.go) rebuilds a virtual day's start from the series. With no StartsAtTimeZone it uses time.UTC, so the series' 14:00 UTC is kept on 2026-11-04 instead of its 9:00 local wall time. The comments in events.go / events_zone.go also describe HEY reading zoneless times as UTC, which matches the API but not the web app.

Suggested fix

• In occurrenceInstants, expand a zoneless series in the HEY account's zone (accountZone), not UTC, so an edit keeps the day where the web app shows it. Consider refusing the edit when the account has no zone, as writeZone does.
• On HEY's side, the period API should expand zoneless series the way the web calendar does, so starts_at is right for every API client.

Environment

• hey 1.9.0 (main at 02efc42), hey-sdk 0.32.1
• macOS, America/Chicago
• Related but separate: Timed events show on their UTC day, not their local day, west of UTC #585 / Show a timed event on its local day, not its UTC day #563 (which local day an event lands on)