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/tests/test_branch_clause.py

@@ -350,6 +350,28 @@ def test_a_memory_written_without_a_branch_is_placed_anyway(forked):
     assert memory.depth == 3
 
 
+def test_placing_a_flush_of_nodes_reads_the_branch_once(forked, emitted_sql):
+    """The guard resolves the head once per flush, not once per node.
+
+    The identity map holds weak references, so a branch row nobody keeps a
+    strong reference to is collected between two nodes and read back for the
+    next one. Writing two hundred actions in one flush was two hundred SELECTs
+    on `branches` before the head was hoisted out of the loop, and nothing
+    about the result would have told you.
+    """
+    db, adventure, _ = forked
+    emitted_sql.clear()
+    for i in range(50):
+        db.add(models.Action(
+            adventure_id=adventure.id, index=500 + i, type="do", text=f"bulk {i}"
+        ))
+    db.commit()
+    branch_reads = [s for s in emitted_sql if s.startswith("SELECT") and "FROM branches" in s]
+    assert len(branch_reads) <= 2, (
+        f"{len(branch_reads)} reads of `branches` to place 50 nodes"
+    )
+
+
 def test_an_adventure_with_no_branch_at_all_reads_as_empty(forked):
     """The loud version of a missing branch: nothing, rather than everything.