Refresh what a switch changes, not what a turn changes

A branch switch does not change the length of the story. It changes which
story it is. Four panels keyed on actions.length and so could not tell the
difference: the Branches panel drew one branch while the reader was already
on a second, Insights showed the prompt built for the path just left, the
script-state drawer kept the other line's numbers, and the Memory Bank did
not notice a deleted branch taking its memories with it.

Only the world-state drawer was right, and only because it happened to
carry stateKey already. They all key on the pair now, and deleting a branch
bumps it too — that is the one operation that changes what is stored
without a turn being played and without the story on the current path
moving by a single action.

tools/branch_fixture.py is the thing that could show it. The stress
fixture's world state is empty, so it cannot answer whether a switch puts
the scoreboard back, and its story is one branch. This builds a small
bootable adventure with a stat schema, a gold script, two takes on one turn
that differ by 35 hit points, a fork, and a memory on each side — with both
branches the same length on purpose, because equal length is precisely the
case a length-based key cannot see.

Verified in a browser with the drawer open: hp 60 to 95 and back, the bar
redrawn, the story swapped to the other take, Insights carrying the scratch
and not the beating. The Memory Bank deliberately does not change on a
switch: the drawer is adventure-wide so a memory is always findable to
delete, and retrieval is the path-scoped half.

396 tests, build clean, no new lint.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015H5qiyiR7gtFQaoDphHZ3g

plan/STATUS.md

@@ -149,13 +149,22 @@ read the same `GET /branches`.
 `rename` had no column and no route; `delete` had no route. Migration 61 adds
 `branches.name`, and the two endpoints came with it.
 
-**One bug, and only a browser could have found it.** The panel refreshed on
-`actions.length`. Forking from the story column swaps a 60-action window for another
-60-action window, so the length never changes — the panel kept drawing a one-branch tree
-while the story was already being read on the second branch. The server was right the
-whole time and no test could see it. That is now three bugs on this frontend found by
-exercising it rather than by testing it, and the second found in a path that had just
-shipped.
+**One bug class, in four places, and only a browser could have found it.** The Branches
+panel, the Status drawer, Insights and the Memory Bank all refreshed on `actions.length`
+— and **a branch switch does not change the length of the story, it changes which story
+it is.** So the tree showed one branch while the reader was on a second, Insights showed
+the prompt for the path just left, and the scoreboard kept the other line's numbers. The
+server was right the whole time and no test could see any of it. All four key on
+`${actions.length}:${stateKey}` now.
+
+`backend/tools/branch_fixture.py` was written to catch exactly this and is worth keeping:
+a small bootable adventure with real stats whose **two branches have equal path length**,
+which is the case a length-based key cannot distinguish. `--keep` could not have found
+it. Run it, switch branches, and watch hp go 60 ↔ 95 with the drawer open.
+
+That is now four bugs on this frontend found by exercising it rather than by testing it,
+two of them in paths that had just shipped. The pattern is not subtle any more: **this
+frontend has no test runner, so anything not driven by hand is unverified.**
 
 **The scroll path is finally driven.** 602-action fixture, three prepends of ~16,200 px
 each: the same DOM node held viewport top 792 → 787, and the view stayed 48,174 px from