Fix G06 500 error: call env.reset() on session creation to init hidden state

G06 (DirectionalFlow) requires orientation to be set before get_true_next()
can be called. Session creation never called reset(), leaving _orientation=None
and causing RuntimeError -> 500 on submit_rule.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -183,6 +183,7 @@ def create_session(body: SessionCreate, _: None = Depends(_auth)):
     except KeyError as exc:
         raise HTTPException(400, str(exc))
 
+    env.reset(seed=42)  # initialises hidden state (e.g. G06 orientation) before any scoring
     sid = str(uuid.uuid4())
     with _lock:
         _sessions[sid] = {