Honest measured pipeline efficiency: two timing fixes

Two related issues caused measured pipeline efficiency to look
worse than the simulator's actual behavior:

1. DMA timing recorded too early. The op-log start timestamp
   for a DMA op fired when the request entered the queue, and
   the DMA channel was released as soon as the request was
   issued. Back-to-back DMAs therefore appeared to grab the
   channel simultaneously, with per-op duration drifting
   upward as queue depth grew - an artifact, not real cost.

   Fix: defer the start timestamp until after the channel is
   acquired, and hold the channel through the full HBM
   round-trip until the response returns. Per-op duration is
   now constant and equal to the actual transfer interval;
   serialization is visible as queue wait, not as inflated
   service time.

2. Sweep timing window folded in pre-composite work. The PE
   timing window spanned every PE engine record, which
   included the upfront pinned-operand DMA issued before the
   composite GEMM begins. For large-K shapes that one-shot
   load can be nearly half of the window, conflating
   operand-staging cost with composite-pipeline behavior.

   Fix: add a second window scoped to the composite pipeline
   by filtering op_log records to those tagged with a
   tile-pipeline stage; the legacy operand-load path is
   untagged and naturally excluded. For 32x3072x32 load_ref
   the window drops from 1765ns to 992ns and measured eff
   lines up with the steady-state DMA-bound stage limit
   instead of being penalized for the one-time load.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/gemm_sweep.py

@@ -179,6 +179,13 @@ def _run_one(M: int, K: int, N: int, topology: str, variant: str = "ref_ref") ->
                               - min(r.t_start for r in pe_records)
     else:
         row["pe_window_ns"] = 0.0
+    stage_records = [r for r in op_log
+                     if r.params.get("stage_type") in STAGES]
+    if stage_records:
+        row["composite_window_ns"] = max(r.t_end for r in stage_records) \
+                                     - min(r.t_start for r in stage_records)
+    else:
+        row["composite_window_ns"] = 0.0
     return row