d2sequence: account for self and sibling message height in note placement

[AdamMagued]

Fixes #2939

Problem

When notes are positioned in sequence diagrams, vertical offsets only accounted for self-messages where source matched destination directly (msg.Src == msg.Dst). Messages between descendant spans or sibling spans sharing the same actor root are routed as multi-point self-loops, which occupy extra vertical space. Because note placement overlooked these messages, notes following sibling messages were positioned prematurely along the Y axis. Consequently, subsequent groups containing or starting with such notes computed overlapping bounding boxes.

Solution

1. Introduce isSelfOrSibling helper to consistently classify self-messages, descendant messages, and sibling messages sharing the same actor root.
2. Use isSelfOrSibling in placeNotes to add the full vertical space occupied by loop routes when calculating vertical offsets.
3. Replace duplicate inline message classification logic in routeMessages with isSelfOrSibling.
4. Add automated unit test coverage in layout_test.go verifying that groups starting with notes after sibling messages do not overlap.