Restructure legacy backups, remove pe_accel, fix DMA self-routing

- Move builtin_legacy/ → legacy/builtin/ (cleaner structure)
- Move pe_accel_legacy/ → legacy/pe_accel/
- Remove custom/pe_accel/ (replaced by new builtin)
- Remove pe_scheduler_v2 from components.yaml
- Switch topology.yaml to pe_scheduler_v1 (new builtin)
- Fix PE_DMA self-routing: handle consecutive DMA_READ stages
  (same component consecutive stages processed in-place, not via port)

382 tests passing.

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

src/kernbench/components/legacy/builtin/forwarding.py

@@ -0,0 +1,27 @@
+from __future__ import annotations
+
+from collections.abc import Generator
+from typing import TYPE_CHECKING
+
+import simpy
+
+from kernbench.components.base import ComponentBase
+
+if TYPE_CHECKING:
+    from kernbench.components.context import ComponentContext
+    from kernbench.topology.types import Node
+
+
+class TransitComponent(ComponentBase):
+    """Transit component for NOC, UCIe, XBAR nodes.
+
+    Applies overhead_ns processing delay (from node.attrs) then forwards the
+    Transaction to the next hop via inherited _forward_txn().
+    """
+
+    def __init__(self, node: Node, ctx: ComponentContext | None = None) -> None:
+        super().__init__(node, ctx)
+
+    def run(self, env: simpy.Environment, nbytes: int) -> Generator:
+        overhead_ns = float(self.node.attrs.get("overhead_ns", 0.0))
+        yield env.timeout(overhead_ns)