feat(orchestrator): soft occupancy threshold requests handoff early (S11 partial)

Coordinator.Soft (default 0.55, ORCHESTRA_OCCUPANCY_SOFT) makes rotate()
and TurnDecision request a handoff advisory-only once occupancy crosses
the soft threshold, well before Hard forces a release/rotation.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W1rkJ2hBMybnJctPbcy4tT

internal/orchestrator/rotation_test.go

@@ -218,6 +218,26 @@ func TestTurnDecision(t *testing.T) {
 		}
 	})
 
+	t.Run("prepare_handoff at soft threshold, below hard, without a turn boundary", func(t *testing.T) {
+		a := &handoffRequestingAdapter{fakeAdapter: fakeAdapter{occupancy: .6, boundary: false}}
+		c, st, task := newCoordinator(&a.fakeAdapter)
+		c.Adapters = adapters{a}
+		decision, err := c.TurnDecision(context.Background(), task.ID)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if decision != orchestrator.TurnPrepareHandoff {
+			t.Fatalf("decision=%q want %q", decision, orchestrator.TurnPrepareHandoff)
+		}
+		if a.requests == 0 {
+			t.Fatal("RequestHandoff was never invoked at the soft threshold")
+		}
+		got, ok := st.Task(task.ID)
+		if !ok || got.State != domain.StateLeased {
+			t.Fatalf("task state=%v ok=%v, want still leased (soft threshold is advisory)", got.State, ok)
+		}
+	})
+
 	t.Run("prepare_handoff requests handoff without releasing", func(t *testing.T) {
 		a := &handoffRequestingAdapter{fakeAdapter: fakeAdapter{occupancy: .95, boundary: true}}
 		c, st, task := newCoordinator(&a.fakeAdapter)