Generative: the embedding turns by drag past its edge; version diffs are tested

The turned embedding turns through gpui's drag, marked with its view, so a
drag keeps turning past the plot's edge. What a chosen version is set
against, and its heading, move into versions::compared, with a test. A
hidden legend entry dims its key and not its ring, and an embedding refuses
a group named twice.

TASKS.md

@@ -301,3 +301,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T25d | 2 | PASS | |
 | T25e | 1 | FAIL | a description row never wrapped, so a 280px model card broke its facts mid-word; the fine-tune bar's share had no test |
 | T25e | 2 | PASS | |
+| T25f | 1 | FAIL | a drag on the turned embedding froze past the plot's edge; which version a diff sets against, and its heading, had no test |