A follower could crash when following another collaborator into a
newly-opened multibuffer if the leader's recent edits hadn't yet
propagated. The follower would receive the view state with excerpt
anchors pointing into still-unobserved edits, tripping
`panic_bad_anchor`.

The fix waits for each buffer to observe the anchors timestamps before
resolving them, matching what's already done for selection and scroll
anchors. Includes a regression test in `collab` that reproduces the race
deterministically.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed a crash in follow mode when opening multibuffers