Compare pins with a target branch and flag pins off it

The ref of a pin was the closest branch containing it, so a pin on a
feature branch looked like a perfect one, and the ref changed with the
branch checked out in the submodule. Split it in two:

- Pin.Target is the branch the pin should be on: `branch =` of
  .gitmodules (none if it does not exist locally), else the remote's
  default branch.
- Pin.Ref is shown: the target if the pin is on it, else the branch
  containing the pin with the fewest commits after it. A tie goes to the
  upstream of the checked-out branch, then to the first name; no branch is
  a favorite.

A pushed pin that is not on its target is `off target`, a new issue at the
new level Info (blue), counted in the header and below collapsed rows. The
spine shows the target's tip, labelled with its branch name, so the
distance from it is visible; a diverged one is the fork. The role words in
a spine label are styled separately and `you` is bold.

Add Status.TargetTip, TargetDrift and Bases.Target, Pin.Target and
Pin.Branches; drop RefSource.IsTarget. The detail pane's label column is 11
wide for the word `off target`.

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

internal/submodule/spine_test.go

@@ -122,6 +122,20 @@ func TestSpine(t *testing.T) {
 			nodes: []string{"t:tip", "p:pin", "y:you"},
 			links: []Link{{Above: 2, Below: 1}, {Above: 1, Below: 1}},
 		},
+		{
+			name: "pin on a branch ahead of its target: the target is below the pin",
+			st: Status{Pinned: c("p"), CheckedOut: c("p"), Tip: c("t"), Pin: PinInfo{Behind: 1},
+				TargetTip: c("m"), TargetDrift: Distance{Behind: 3}},
+			nodes: []string{"t:tip", "p:pin,you", "m:target"},
+			links: []Link{{Above: 1}, {Above: 3}},
+		},
+		{
+			name: "pin on a branch diverged from its target: a fork",
+			st: Status{Pinned: c("p"), CheckedOut: c("p"), Tip: c("t"), Pin: PinInfo{Behind: 1},
+				TargetTip: c("m"), TargetDrift: Distance{Ahead: 12, Behind: 52}, Bases: Bases{Target: c("b")}},
+			nodes: []string{"m:target", "t:tip", "p:pin,you", "b:base"},
+			links: []Link{{Above: 12}, {Above: 1}, {Above: 52}},
+		},
 		{
 			name:  "no tip, checkout not loaded",
 			st:    Status{Pinned: c("p")},
@@ -134,7 +148,7 @@ func TestSpine(t *testing.T) {
 			links: []Link{{Above: 2}},
 		},
 	}
-	words := map[Role]string{RolePin: "pin", RoleStaged: "staged", RoleHead: "head", RoleTip: "tip", RoleYou: "you", RoleBase: "base"}
+	words := map[Role]string{RolePin: "pin", RoleStaged: "staged", RoleHead: "head", RoleTip: "tip", RoleTarget: "target", RoleYou: "you", RoleBase: "base"}
 	for _, tt := range tests {
 		var nodes []string
 		var links []Link