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>

scripts/make-fixture.sh

@@ -44,7 +44,7 @@ upstream() {
 mkdir -p "$root/remotes" "$root/work"
 for r in core ui net store log feature nested work; do remote "$r"; done
 
-# "feature" has a branch that main doesn't contain
+# "feature" has a branch that main doesn't contain (a pin there is off target)
 g "$root/work/feature" switch -c feat/search
 commit "$root/work/feature" search "Add search"
 commit "$root/work/feature" search "Tune search"
@@ -72,7 +72,7 @@ g "$w" push origin main
 g "$root/work/log" submodule add "$root/remotes/nested.git" modules/nested
 g "$root/work/log" commit -m "Add nested"
 g "$root/work/log" push origin main
-# "archive" ties with main (same tip, sorts first) -> the preferred ref wins
+# "archive" contains the pin of log like main (same tip): another branch with it
 g "$root/work/log" push origin main:archive
 
 super="$root/super"
@@ -113,7 +113,7 @@ g "$super" add libs/feature
 g "$super" commit -m "Pin feature search"
 
 # libs/topic: .gitmodules says main, but the pin is the tip of feat/search
-# -> the closest branch wins over the configured one
+# -> off target: the pin is on a feature branch, not on the configured one
 g "$super/libs/topic" fetch
 g "$super/libs/topic" checkout origin/feat/search
 g "$super" add libs/topic