Split Play.jsx into a directory of twelve files

`frontend/src/pages/Play.jsx` was 2280 lines holding 27 components. It is now
`frontend/src/pages/Play/`: `index.jsx` with the page component, five panels,
two drawers, the reports, the take pager, the refresh dialog, and the shared
formatting helpers.

Every line moved verbatim. A coverage check confirms every non-blank line of
the original appears exactly once, in order, across the twelve files, and a
name-resolution check confirms every identifier each file references is defined
or imported there, with no unused imports.

The line split stranded a comment at six of the boundaries. A leading comment
sits above the section it describes, so each cut left one at the end of the file
before it. All six moved to the section they describe, rewritten in the house
style.

`usePlaySession.js` is not here. The page component still owns all of the
session state. Moving eighteen `useState` calls and seven `useEffect` calls is a
rewrite rather than a move, and no frontend test would catch a mistake in it
today, so it waits for Stage 5.

Four stand-in providers under `backend/tools/` now define `last_usage = None`.
The turn engine reads that attribute after every call, and the fixtures never
defined it, so `tools.tree_fixture` crashed. That break predates this branch.

Verified by 549 passing tests, a clean `npm run lint` and `npm run build`, and
by driving the Play screen: the story view, all five panels, both drawers
including the world-state edit form, the branch map, the refresh dialog, and the
take pager stepping onto a take that lives on another branch. No console errors.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014Dix4oGV3njgWRdu7P9t6r

backend/tools/stress_session.py

@@ -196,6 +196,10 @@ class FakeProvider:
     time.
     """
 
+    # The turn engine records the cost of the call, so a stand-in provider has
+    # to carry this attribute even when it never calls anything.
+    last_usage = None
+
     def __init__(self, *a, **k):
         pass