Rename impl names: add builtin. prefix for clear provenance

- components.yaml: all builtin impls use builtin.xxx naming
- topology.yaml: all impl references updated to builtin.xxx
- builder.py: hardcoded ucie impl → builtin.ucie
- Tests: all impl string references updated

Convention: builtin.<name> for built-in, custom.<name> for user-defined.
382 tests passing.

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

tests/test_component_registry.py

@@ -55,7 +55,7 @@ def test_registry_unknown_impl_raises_error():
 
 def test_transit_component_yields_overhead_ns():
     """TransitComponent.run() yields exactly node.attrs['overhead_ns'] ns."""
-    node = _node("forwarding_v1", overhead_ns=3.0)
+    node = _node("builtin.forwarding", overhead_ns=3.0)
     comp = TransitComponent(node)
     env = simpy.Environment()
 
@@ -69,7 +69,7 @@ def test_transit_component_yields_overhead_ns():
 
 def test_transit_component_zero_overhead_ns():
     """TransitComponent with overhead_ns=0 still yields (no infinite loop)."""
-    node = _node("noc_v1", overhead_ns=0.0)
+    node = _node("builtin.noc", overhead_ns=0.0)
     comp = TransitComponent(node)
     env = simpy.Environment()
 
@@ -100,7 +100,7 @@ def test_engine_component_override_is_called():
 
     SpyXbar.calls = 0
     graph = _graph()
-    engine = GraphEngine(graph, component_overrides={"forwarding_v1": SpyXbar})
+    engine = GraphEngine(graph, component_overrides={"builtin.forwarding": SpyXbar})
     msg = MemoryReadMsg(
         correlation_id="c", request_id="r",
         src_sip=0, src_cube=0, src_pe=0,
@@ -108,7 +108,7 @@ def test_engine_component_override_is_called():
     )
     h = engine.submit(msg)
     engine.wait(h)
-    # Path passes through router nodes (impl=forwarding_v1)
+    # Path passes through router nodes (impl=forwarding)
     assert SpyXbar.calls > 0
 
 
@@ -140,7 +140,7 @@ def test_engine_component_model_latency():
 
 
 def test_engine_override_is_scoped_to_impl():
-    """forwarding_v1 override (ZeroRouter, no overhead) reduces total_ns.
+    """forwarding override (ZeroRouter, no overhead) reduces total_ns.
 
     Router nodes have overhead_ns=2.0. Replacing with zero-latency impl
     removes router overhead from the path.
@@ -152,7 +152,7 @@ def test_engine_override_is_scoped_to_impl():
 
     graph = _graph()
     engine_default = GraphEngine(graph)
-    engine_override = GraphEngine(graph, component_overrides={"forwarding_v1": ZeroRouter})
+    engine_override = GraphEngine(graph, component_overrides={"builtin.forwarding": ZeroRouter})
 
     msg = MemoryReadMsg(
         correlation_id="c", request_id="r",
@@ -168,5 +168,5 @@ def test_engine_override_is_scoped_to_impl():
     engine_override.wait(h_o)
     _, t_override = engine_override.get_completion(h_o)
 
-    # ZeroRouter removes overhead from all forwarding_v1 nodes in path.
+    # ZeroRouter removes overhead from all forwarding nodes in path.
     assert t_override["total_ns"] < t_default["total_ns"]