gemm: test-generated GEMM plots under tests/gemm/ + docs/diagrams/gemm_plots/

Mirror the sccl pattern for GEMM figures: a tests/gemm/ package renders the
GEMM bar charts as PNGs from the committed docs/diagrams/gemm_sweep.json, so
the figures are fast test artifacts (run by default) while the heavy sim sweep
stays a manual script (scripts/gemm_sweep.py, kept) wrapped by a slow
regenerator test.

tests/gemm/:
- _gemm_plot_helpers.py: matplotlib renderers (series logic mirrors the
  GEMM _render_* functions in scripts/build_overview_slides.py).
- test_plot_gemm_stage_breakdown.py: gemm_stage_breakdown.png (load_ref).
- test_plot_gemm_mac_utilization.py: gemm_mac_utilization_measured.png +
  gemm_mac_utilization_theoretical_vs_measured.png (load_ref).
- test_gemm_sweep.py: @pytest.mark.slow regenerator (runs scripts/gemm_sweep.py).

Chart set trimmed to three (stage breakdown, MAC util, theoretical-vs-measured);
"formula" relabeled to "theoretical" throughout the comparison chart.

Known follow-ups (not blocking):
- gemm_mac_utilization_measured.png currently plots the theoretical ideal-
  pipeline model, not simulator-measured data; the name is a misnomer pending
  a decision to repoint its content or retitle.
- The theoretical-model constants (HBM 256 GB/s, T_stage 16 ns, 3 stages) are
  inherited verbatim from build_overview_slides.py and not yet verified against
  ADR-0033 / ADR-0014 / topology.

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

tests/gemm/test_plot_gemm_mac_utilization.py

@@ -0,0 +1,35 @@
+"""Emit the GEMM MAC-utilization bar charts.
+
+A measured chart (load_ref) plus the theoretical-vs-measured overlay (load_ref).
+Reads docs/diagrams/gemm_sweep.json and writes gemm_mac_utilization*.png into
+docs/diagrams/gemm_plots/.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from tests.gemm._gemm_plot_helpers import (
+    GEMM_SWEEP_JSON,
+    emit_mac_utilization_measured,
+    emit_mac_utilization_theoretical_vs_measured,
+)
+
+
+@pytest.mark.skipif(
+    not GEMM_SWEEP_JSON.exists(),
+    reason="gemm_sweep.json absent; run scripts/gemm_sweep.py first",
+)
+def test_plot_gemm_mac_utilization_measured():
+    out = emit_mac_utilization_measured()
+    assert out is not None and Path(out).exists()
+
+
+@pytest.mark.skipif(
+    not GEMM_SWEEP_JSON.exists(),
+    reason="gemm_sweep.json absent; run scripts/gemm_sweep.py first",
+)
+def test_plot_gemm_mac_utilization_theoretical_vs_measured():
+    out = emit_mac_utilization_theoretical_vs_measured()
+    assert out is not None and Path(out).exists()