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>

docs/ux-guide.md

@@ -82,7 +82,7 @@ For the superproject (header selected), the `pin` and `you` roles become
 1. Directory (muted) and name (bold); `fetched <age>` of the submodule,
    muted, right-aligned in the name's line (left out if the pane is too
    narrow, or the submodule is missing)
-2. One line per issue: word (colored, 10 columns) + explanation;
+2. One line per issue: word (colored, 11 columns) + explanation;
    `in sync  nothing to do` if there are none and no pin is staged;
    `staged` and `detached HEAD on pin` as information
 3. `Changes` (only with uncommitted changes), see
@@ -107,7 +107,8 @@ you       ● 48005ac Merge branch 'feat/ethercat…
   position, not by comparing SHAs. The commits between two nodes are not
   listed, only counted (`┊ 4 commits`; `1 commit` if adjacent).
 - The label column holds the roles (`pin`, `tip`, `you`; `HEAD` for the
-  superproject), joined by ` · ` if they sit on one commit
+  superproject; the target's tip is labelled with the name of its branch, e.g.
+  `marco-dev`, without the remote and cut at 16 columns; `target` if unknown), joined by ` · ` if they sit on one commit
   (`pin · tip`, `pin · you`). If all of them sit on one commit it says
   `in sync` instead, which also keeps the commit column in line with the
   other values; the column only grows for a longer label.
@@ -116,8 +117,25 @@ you       ● 48005ac Merge branch 'feat/ethercat…
   visible as a distance between two nodes. The committed pin is left out if
   it is not on disk. If the staged pin is *older* than the committed one it
   sits below it, and the gap above it says `(staged pin moves back)` (blue).
-- The checkout (`you`) is yellow (label and SHA) when it is on its own node,
-  i.e. off pin. Everything else is the default text color, the gap muted.
+- If the pin is `off target`, the target branch's tip is on the spine,
+  labelled with the name of its branch (`marco-dev`, without the remote, cut
+  at 16 columns), so the distance from main is visible: below the pin if the pin is
+  on top of the target, a fork if both have commits of their own. `tip` is
+  then the tip of the branch the pin is on (the `Ref`):
+
+  ```
+  marco-dev    ● d0bbbd7 updated submodules
+               ┊ 12 commits
+  tip        ● ┊ a523224 Remove stale entry
+             ┊ ┊ 1 commit
+  pin · you  ● ┊ 5a7df75 Merge branch 'ethercat…
+             ┊ ┊ 52 commits
+  base       ●─╯ 6707725 updated submodule
+  ```
+- The role words are muted, each word on its own: `you` is bold in the
+  default text color, so the checkout stands out in `pin · you`. On its own
+  node, i.e. off pin, it is yellow (word and SHA); `staged` is blue. Commit
+  subjects use the default text color, the gap is muted.
 - A commit that has **diverged** from the pin gets its own lane to the right
   of the main one. The two lanes join at a `base` node, the commit where the
   histories split; the counts say how many commits each side has since:
@@ -198,7 +216,7 @@ branch and state stay at the left. If the line is too narrow it is cut with
 `…` at the right end, so the most important counts stay.
 
 Issue counts are ordered by severity (unpushed, conflict, missing, off pin,
-changed), each in its level color. Zero counts are omitted. `n loading`
+changed, off target), each in its level color. Zero counts are omitted. `n loading`
 counts only submodules whose status is still being loaded; ones whose load
 failed are counted separately as `n error` (red, before the issue counts). A detached
 superproject shows `(detached)` as its branch.
@@ -224,7 +242,7 @@ colors in components.
 |---|---|---|
 | Danger | ANSI `1` red, bold | `unpushed`, `conflict`, `error`, `↑n` of an unpushed pin |
 | Warn | ANSI `3` yellow | `missing`, `off pin`, `changed`, differing checkout SHA |
-| Info | ANSI `4` blue | `staged`, the `●` staged marker |
+| Info | ANSI `4` blue | `staged`, the `●` staged marker, `off target` |
 | Primary | ANSI `6` cyan | branch names, commit hashes, spinner |
 | Muted | ANSI `8` grey | quiet rows, directory headers, arrows, labels |
 | Text | terminal default | normal text |
@@ -248,6 +266,7 @@ colors in components.
 | `missing` | not initialized / directory missing |
 | `off pin` | checkout differs from pin |
 | `changed` | uncommitted changes / untracked files |
+| `off target` | pin not on `<target>`, on `<branch>` (+n more) |
 | `staged` | pin change staged, not committed |
 | `in sync` | nothing to do |