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>

docs/architecture.md

@@ -114,7 +114,8 @@ The mock provider (`internal/submodule/mock`) covers every state and sleeps
   `Status` calls run at once; the others wait on a semaphore and still
   respect `ctx` (D16).
 - Superproject: branch from `symbolic-ref`, ahead/behind against
-  `@{upstream}`, "fetched" from the mtime of `FETCH_HEAD`.
+  `@{upstream}`. Its fetch age (mtime of `FETCH_HEAD`) is `Status.FetchedAt`
+  of the root node, like a submodule's.
 - `Fetch`: `git fetch --all --prune --no-recurse-submodules --quiet` (we
   fetch every submodule ourselves), at most 8 at once, 2 min timeout each.
   To never prompt, `GIT_SSH_COMMAND="ssh -o BatchMode=yes"` is set unless