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/decisions.md

@@ -18,7 +18,7 @@ transparent.
 pin, so a mismatch was invisible.
 **Decision.** Model all three commits. The tree's ref column always describes
 the **pin**; checkout drift is an issue (`off pin`).
-**Consequences.** The detail pane shows Pinned / Checkout / Tip rows.
+**Consequences.** The detail pane shows all three (see D25).
 
 ## D3 — Raw dimensions in the domain, summarized issues in the UI
 
@@ -303,8 +303,8 @@ not issues.
 submodules that is hundreds of network round trips (and credential or
 host-key failures) every time the tool opens, whether or not the user wants
 fresh remote data.
-**Decision.** Supersedes D18. Nothing is fetched on startup. The header and
-the detail pane show the last fetch (`FETCH_HEAD`, also from fetches made in
+**Decision.** Supersedes D18. Nothing is fetched on startup. The detail pane
+shows the last fetch of the selected repository (`FETCH_HEAD`, also from fetches made in
 other shells), so the user can judge how stale the remote refs are and press
 `f` to fetch. The `--no-fetch` flag is removed with it (it still had the
 side effect of switching off the update check; that has
@@ -386,3 +386,31 @@ restores below, without staging.
 **Consequences.** `Switcher` is a new optional provider interface; the mock
 does not implement it. The switch runs as the restore does (one write
 operation at a time, same lines above the footer).
+
+## D25 — Detail pane: a spine instead of three commit rows and a list
+
+**Context.** `Pinned` / `Checkout` / `Tip` were three rows of SHAs to
+compare, the distance sat on the `Ref` line, and a list of up to 50 commits
+(two lines each) pushed the rest out of view. Where the pin, the checkout and
+the tip are had to be worked out by reading.
+**Decision.** Draw them as one line of history, newest on top: a node per
+commit, the roles in the label column, the number of commits between two
+nodes in a gap row. Roles on one commit share a node. The commit list is
+dropped (no cap, no popup): the gap count says how far, not what changed.
+The `Ref` line loses the distance suffix (the gap says it) and keeps
+`via <rule>`.
+**Consequences.** `Status.Between` is gone; `Status.Drift` (checkout ↔ pin,
+one `rev-list --left-right --count`, only when off pin) and `Status.Spine()`
+are new. Author and age of commits are no longer shown anywhere in the pane.
+Diverged tip or checkout show `diverged ↑n ↓m` in the gap; drawing a real
+fork needs the merge base and is left for later.
+
+**Amendment (with D25).** The header no longer shows `fetched <age>`. It was
+the superproject's own fetch, which the tree's `↓n` do not depend on, and read
+as the age of everything. The age lives only in the detail pane, right-aligned
+in the name's line (the superproject's when the header is selected); the
+header keeps the progress and `n failed`. `Repo.FetchedAt` is gone.
+
+**Amendment (with D25).** A staged pin shows both pins on the spine: `staged`
+(the index) and `pin` (the superproject's HEAD, `Status.Committed`), so a
+bump reads as a distance. Before, only the index pin was shown at all.