Extract fusion into GraphPass plugin API

Move ~600 lines of operator fusion code out of subgraph.rs into two new
modules: graph_pass.rs defines a GraphPass trait that receives children
and edges for in-place transformation, and fusion.rs implements it with
the existing detect_groups/fuse_group/GroupScheduler logic.

SubgraphBuilder gains a graph_passes Vec and add_graph_pass() method.
The build() method runs registered passes sequentially before building
the reachability tracker. worker.rs and child.rs register FusionPass
when fuse_chain_length >= 2.

PerOperatorState fields are now pub(crate) so the fusion module can
access them. The forward_to field and tombstone mechanism remain as a
general consequence of graph pass output, not fusion-specific.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antiguru

timely/src/dataflow/scopes/child.rs

@@ -148,7 +148,17 @@ where
             };
             func(&mut builder)
         };
-        let subscope = subscope.into_inner().build(self);
+        let mut subscope = subscope.into_inner();
+
+        // Register the fusion pass if enabled.
+        let fuse_chain_length = self.parent.config().fuse_chain_length;
+        if fuse_chain_length >= 2 {
+            subscope.add_graph_pass(Box::new(
+                crate::progress::fusion::FusionPass::new(fuse_chain_length)
+            ));
+        }
+
+        let subscope = subscope.build(self);
 
         self.add_operator_with_indices(Box::new(subscope), index, identifier);