calendar: compare-on-open — detect edits made outside the app and send them for review (#177) #192

Part of the option-4 plan (#177, decisions on Q4–Q7): an edit to a synced row, made anywhere (the plain table, the host's built-in Calendar view, another view or another device), should reach Google Calendar.

Plan for the calendar drive app:

• Compare on open. When the app opens, compare each synced row's shared fields (atomic-calendar-*, title, notes, per calendar: use the host's calendar field names so all-day and multi-day events render in the Calendar view #172) with the sync-baseline stored on that row. Any difference becomes a pending change.
• Review. Pending changes go into the app's existing "review and send" list, which already sends with If-Match on the stored google-etag. Every send is reviewed while testing; that default is per the decision on Q5.
• Conflicts. If Google also changed since the baseline, it's a conflict, handled the way the app handles one today (a 412, then review).
• Bookkeeping stays on the row as provider extras: google-event-id, google-etag, sync-baseline.
• Deletions. A synced row deleted locally isn't detected until the host's per-table change list exists (Design: plugins sync into shared row classes, and plugin views work on any table of that class ("option 4") #177 H6). That's a declared limitation.

It depends on #172 (calendar 0.1.1, calendarFields), which is being rebased onto main after #181, #183, #184 and #187.

Claimed by the atomic-plugins worker session (e6ce43). If you're working on calendar from another machine, please comment here before building this, to avoid duplicate work. The user-testing log hook (#183) won't be changed.