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>

internal/submodule/model.go

@@ -15,7 +15,6 @@ type Repo struct {
 	Branch     string
 	Ahead      int
 	Behind     int
-	FetchedAt  time.Time
 	Root       *Submodule // the superproject as a selectable node; Path is "", no Children
 	Submodules []*Submodule
 }
@@ -173,6 +172,10 @@ func (c *Changes) Sort() {
 	})
 }
 
+// Distance counts the commits only one side of a comparison has, like
+// `git rev-list --left-right --count a...b`.
+type Distance struct{ Ahead, Behind int }
+
 // Status is the full state of one submodule.
 type Status struct {
 	Root       bool // the superproject: the "pin" is its HEAD
@@ -184,7 +187,12 @@ type Status struct {
 	Pinned     *Commit
 	CheckedOut *Commit
 	Tip        *Commit
-	Between    []Commit  // commits that make up the pin <-> tip difference
-	Changes    *Changes  // nil if the worktree is clean or the details are unknown
-	FetchedAt  time.Time // last fetch of the submodule; zero if never
+	Drift      Distance // checkout compared with the pin: Ahead = commits only the checkout has
+	// Committed is the pin in the superproject's HEAD while Pinned (the index) is
+	// staged; nil if nothing is staged, the commit is not on disk, or the pin was
+	// only added. CommittedDrift compares it with Pinned like Drift.
+	Committed      *Commit
+	CommittedDrift Distance
+	Changes        *Changes  // nil if the worktree is clean or the details are unknown
+	FetchedAt      time.Time // last fetch of the submodule; zero if never
 }