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>

docs/state-model.md

@@ -31,7 +31,8 @@ in several at once.
 | `PinStaged` | Pin changed in the superproject index but not committed |
 | `Pin` | `Ref`, `Source`, `Ahead`, `Behind`, `Pushed` |
 | `Pinned`, `CheckedOut`, `Tip` | Commits (SHA, subject, author, time) |
-| `Between` | Commits that make up the pin ↔ tip difference |
+| `Drift` | Checkout compared with the pin: `Ahead` = commits only the checkout has, `Behind` = commits only the pin has |
+| `Committed`, `CommittedDrift` | With a staged pin: the pin in the superproject's HEAD (nil if it is not on disk or the submodule was only added) and how it compares with the staged pin, like `Drift` |
 | `Changes` | Changed files of the worktree (nil if clean), see below |
 | `FetchedAt` | Last fetch of the submodule (`FETCH_HEAD` mtime); zero if never |
 
@@ -172,7 +173,8 @@ are local; `<pin>` is the index gitlink.
 | `Pushed` | `for-each-ref --contains <pin> refs/remotes` is not empty |
 | `Pin.Ref` | rules above; the remote is `origin`, else the first remote |
 | `Ahead`, `Behind` | `rev-list --left-right --count <pin>...<ref>` |
-| `Between` | `log <pin>...<ref>`, at most 50 commits |
+| `Drift` | `rev-list --left-right --count <head>...<pin>`, only if the checkout differs from the pin |
+| `Committed`, `CommittedDrift` | `ls-tree HEAD` gitlink, if it differs from the pin and exists locally; `rev-list --left-right --count <committed>...<pin>` |
 
 Details:
 
@@ -196,6 +198,29 @@ Details:
   fails with "pinned commit … not fetched" (shown as `error`) instead of
   guessing `unpushed`.
 
+## Spine
+
+`Status.Spine()` arranges the pin, the tip and the checkout as one line of
+history, newest on top, for the detail pane. Each node is a commit with its
+roles (`pin`, `tip`, `you`; `HEAD` for the superproject). It uses `Pin.Ahead` /
+`Pin.Behind` (pin ↔ tip) and `Drift` (checkout ↔ pin).
+
+- Roles on the same commit share a node (`pin · tip · you`);
+  `SpineNode.InSync()` is true if all of them are on it (`HEAD` and `tip` for
+  the superproject).
+- Tip or checkout *ahead* of the pin (it is their ancestor) sit above it,
+  *behind* below it; the `Link` between two nodes counts the commits
+  separating them. The count is the distance, so a tip 4 commits ahead
+  of the pin shows `4 commits`, and an adjacent one `1 commit`.
+- A diverged tip is placed above the pin, a diverged checkout below it,
+  with a diverged `Link` (`Above` and `Below` count the commits only the
+  upper or lower node has). The link between a diverged node and a node other
+  than the pin is unknown (zero `Link`).
+- With a staged pin the node of `Pinned` (the index) is labelled `staged`, and
+  the committed pin gets its own `pin` node, placed by `CommittedDrift` like
+  the checkout. Both can share nodes with `tip` and `you`.
+- No tip (the pin resolved to a SHA) or no checkout commit: no node.
+
 ## Distance notation
 
 | Display | Meaning |