Put the pager back, and let any turn be played again

SP7 replaced the pager with chips, on the grounds that a chip could also offer
"take this path" while a pager could only step. Driving it by hand said
otherwise, and the reason is worth keeping: the chip meant two different
things depending on where the reader was standing -- a real switch at the tip,
a preview needing a second button above it further back. Two meanings in one
control is what made the tree unusable.

So: one control that does one thing. Stepping reads a take and nothing else,
and it tells the server nothing, because reading is not a decision. The
transcript below a take that is not live simply ends -- such a take is a leaf
by construction, since whatever was played after the turn was played after the
take that *is* live. The decision is made by writing, and `after_id` carries it.

One step does reach the server and is still not a fork: a take with a story of
its own lives on its own branch, so going there is a branch switch and only the
server can say what is underneath. `branch_id` on the take is what tells the
two apart without asking first.

And a fork button on every turn but the opening. On the AI's it regenerates; on
your own it opens the text so you can say something else. What the story made
of the old take is kept, on the line it was written on.

`selectVariant` and `forkFromAttempt` leave the client. Both endpoints stay --
tested, and `stand_on` is shared with the write path -- but the pager needs
neither.

426 backend tests; lint and build clean. Not yet driven by hand: the frontend
still has no test runner, so this needs the `--keep` fixture and eyes, exactly
as SP7 did.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Dvvqn9ZDR4ixeFPHNbww7

backend/app/routers/adventures.py

@@ -46,8 +46,10 @@ ACTION_LIST_COLUMNS = (
     models.Action.variant_index,
     # SP9: the pager's key. Deferred, it would be a lazy load per row — a query
     # behind every message on the page, which is the whole thing `load_only`
-    # is here to stop.
+    # is here to stop. `branch_id` rides along for the same reason: the pager
+    # reads it to tell a local step from a branch switch.
     models.Action.parent_id,
+    models.Action.branch_id,
     models.Action.created_at,
 )
 
@@ -1014,6 +1016,7 @@ def list_variants(
             index=i,
             text=row.text,
             reasoning=row.reasoning,
+            branch_id=row.branch_id,
             created_at=row.created_at.isoformat() if row.created_at else None,
             active=row.live,
         )