Implement ADR-0021: PE pipeline refactor with token self-routing

Step 1-2: Backup existing code
- builtin/ → builtin_legacy/ (unchanged backup)
- custom/pe_accel/ → custom/pe_accel_legacy/ (unchanged backup)

Step 3-4: New pipeline types and tiling
- pe_types.py: StageType, Stage, TilePlan, PipelinePlan, PipelineContext, TileToken
- tiling.py: generate_gemm_plan, generate_math_plan (ported from pe_accel)

Step 5: Component implementations (ADR-0021 D4-D6)
- PE_SCHEDULER: _feed_loop (singleton FIFO feeder) + plan generation
- PE_FETCH_STORE: new component — TCM ↔ Register File
- PE_GEMM: TileToken pipeline + legacy PeInternalTxn dual-mode
- PE_MATH: TileToken pipeline + legacy dual-mode
- PE_DMA: TileToken pipeline + legacy + fabric Transaction triple-mode
- PE_TCM: TcmRequest handler with dual-channel BW serialization

Step 6: Infrastructure
- topology.yaml: pe_fetch_store component + chaining edges
- components.yaml: pe_fetch_store_v1 registration
- builder.py: PE_COMP_OFFSETS, _add_pe_internal_edges, PE view positions
- Tests: node/edge counts, PE component sets updated

All components handle both TileToken (pipeline) and PeInternalTxn (legacy).
Token self-routing: components read next stage from token.plan, chain via out_port.
366 tests passing.

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

src/kernbench/components/builtin_legacy/pe_tcm.py

@@ -0,0 +1,25 @@
+from __future__ import annotations
+
+from collections.abc import Generator
+from typing import TYPE_CHECKING
+
+from kernbench.components.base import ComponentBase
+
+if TYPE_CHECKING:
+    from kernbench.components.context import ComponentContext
+    from kernbench.topology.types import Node
+
+
+class PeTcmComponent(ComponentBase):
+    """PE_TCM: tightly-coupled memory / local SRAM staging buffer.
+
+    Terminal storage component for PE-internal dataflow (ADR-0014 D5).
+    Phase 0: applies overhead_ns and drain_ns at terminal.
+    """
+
+    def __init__(self, node: Node, ctx: ComponentContext | None = None) -> None:
+        super().__init__(node, ctx)
+
+    def run(self, env, nbytes: int) -> Generator:
+        overhead_ns = float(self.node.attrs.get("overhead_ns", 0.0))
+        yield env.timeout(overhead_ns)