Show the pin, tip and checkout as a spine in the detail pane

Replace the Pinned/Checkout/Tip rows and the list of commits between pin
and tip with one line of history, newest on top: a node per commit with
its roles (pin, tip, you, staged, HEAD) and the number of commits between
two nodes. A staged pin shows the committed pin as its own node and notes
when it moves back. All three on one commit read "in sync".

Add Status.Drift, Committed and CommittedDrift and Status.Spine(); drop
Status.Between and Repo.FetchedAt. The fetch age moves from the header to
the detail pane (right-aligned in the name's line). The header summary is
right-aligned, ordered by importance and cut with an ellipsis if too narrow.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -59,8 +59,8 @@ sub --mock           # built-in sample data, no repository needed
 
 Problems stand out: `unpushed` in red, `off pin` and `changed` in yellow,
 everything else muted. Select a submodule to see the details on the right:
-every issue in words, the resolved ref and why, the pinned, checked-out and
-tip commits, and a preview of uncommitted changes.
+every issue in words, the resolved ref and why, where the pin, the tip and
+your checkout are in the history, and a preview of uncommitted changes.
 
 ```
  src/                                    │ src/
@@ -68,11 +68,10 @@ tip commits, and a preview of uncommitted changes.
    ▸ net [2]     unpushed  origin/main ↑1│
      ui          in sync   origin/main   │ unpushed  pin not pushed to any remote
                                          │
-                                         │ Ref       origin/main ↑1
-                                         │           via checked out branch
-                                         │ Pinned    b7c8d9e Hotfix null pointer
-                                         │ Checkout  b7c8d9e Hotfix null pointer
-                                         │ Tip       6f5e4d3 Release 2.1.0
+                                         │ Ref       origin/main · via checked out branch
+                                         │ pin · you ● b7c8d9e Hotfix null pointer
+                                         │           ┊ 1 commit
+                                         │ tip       ● 6f5e4d3 Release 2.1.0
 ```
 
 ### Jump to a submodule with `/`