Document that time-log's lost users are not sent, and decide whether to warn #49

Disclaimer: AI-generated text below the line, generated by Claude Opus 5.5, from the prompt:

merge #46 as for now [found by the clean-context review of that merge, and filed rather than fixed]

Since #46, when a collection declares time-log as lossy with lost: ["users"] (servers refusing an event that holds a PARTICIPANT: CCS, GMX), the generic layer leaves the users out of what it sends (src/calendaring/_async/collection.py, around line 286). Before that, lost only described what a backend drops by itself.

Two things to settle:

• docs/design/API_DESIGN.md (the time-log paragraph, around line 988) should say that the client strips declared-lost users before sending.
• log_time() and amend_time() return users=() without the loss policy or any warning firing, unlike item writes. Should they go through the loss policy?