multibuffer: Speed up anchor resolution by avoiding an excerpts seek

[cole-miller]

Before, every creation of AnchorSeekTarget called MultiBufferSnapshot::buffer_for_path, which seeks the excerpts sum tree. Now we avoid that by

• looking up the buffer snapshot for the anchor's buffer in the buffers map (keyed by BufferId)
• handling the case where the anchor's buffer doesn't exist at the original path key with a separate AnchorSeekTarget::Missing variant

We also added a separate optimization to AnchorSeekTarget::cmp, to skip the full PathKey comparison when the cursor's PathKeyIndex is the same as the anchor's. This should help with singleton multibuffers.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• N/A

[ConradIrwin]

Makes sense to me, thanks!