Resolve the head once a flush, not once a node

The identity map holds weak references. A branch row nobody keeps a strong
reference to is collected between two nodes, so resolving the head inside the
placement loop read it back from the database for every node in the flush: 201
SELECTs on `branches` to write 200 actions, and 36 s -> 45 s on the same 297
tests. Nothing about any result changed, which is why only a stopwatch found
it, and why there is now a test counting the reads.

Also: the two reads left un-pathed on purpose say so where they live —
`max_action_index` allocates the legacy `index` and must stay adventure-wide
or two branches issue the same number, and export is a flat v1 bundle whose
reader has no idea branches exist. And `Adventure.actions` keeps its `index`
ordering, because ordering the collection by depth would not make it a story:
it is every branch's actions, and a path is a selection out of it.

318 tests green.

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

backend/app/models.py

@@ -134,6 +134,12 @@ class Adventure(Base):
     story_cards: Mapped[list["StoryCard"]] = relationship(
         back_populates="adventure", cascade="all, delete-orphan"
     )
+    # Every action of the adventure — that is, every *branch's*. Not the story
+    # being played, and re-ordering it by depth would not make it one: the
+    # collection is the tree, and a path is a selection out of it. Anything
+    # showing a reader a story goes through `context.history`, which goes
+    # through the branch clause. What is left here is ownership and the
+    # delete-orphan cascade, which are facts about the adventure.
     actions: Mapped[list["Action"]] = relationship(
         back_populates="adventure",
         cascade="all, delete-orphan",