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>

README.md

@@ -23,10 +23,11 @@ the questions you actually have:
   issue word). A collapsed parent counts the problems hidden below it
   (`·1`) and shows `●` if a staged pin is hidden.
 - **Words, not symbols.** Every state is a short word (`unpushed`, `conflict`,
-  `missing`, `off pin`, `changed`), so nothing depends on color alone.
+  `missing`, `off pin`, `changed`, `off target`), so nothing depends on color
+  alone.
 - **Live.** Follows changes you make in other shells within about two
-  seconds. It never fetches on its own: the header shows when you last did,
-  `f` fetches everything.
+  seconds. It never fetches on its own: the detail pane shows when a
+  repository was last fetched, `f` fetches everything.
 - **Fix it from the same screen.** Restore a repository to exactly what git
   records (`x`), or switch it to another branch (`b`). Both ask first if work
   would be lost.
@@ -58,7 +59,8 @@ sub --mock           # built-in sample data, no repository needed
 ### Find what needs attention
 
 Problems stand out: `unpushed` in red, `off pin` and `changed` in yellow,
-everything else muted. Select a submodule to see the details on the right:
+`off target` (the pin is not on the branch it should be, usually `main`) in
+blue, everything else muted. Select a submodule to see the details on the right:
 every issue in words, the resolved ref and why, where the pin, the tip and
 your checkout are in the history, and a preview of uncommitted changes.