gemm(bench): composite-vs-async dispatch comparison harness + single-op-model sweep

Adds the user-orchestrated async GEMM variants used in paper §3.4 to
contrast with the composite (load_ref) path:
  - matmul-async              : naive (tl.load full A+B, one tl.dot)
  - matmul-async-chunked      : depth-inf prefetch (all B-tiles queued)
  - matmul-async-chunked-db   : depth-2 double-buffer (TCM-bounded)
plus scripts/paper/paper_plot_gemm_async_vs_composite.py (4-way per-PE
TFLOPS sweep + figure) and the regenerated outputs under
1H_milestone_output/gemm/.

Sweep regenerated under the ADR-0064 D8 single-op cost model (commit
2d8271c). At K=3072 the composite-vs-async-tiled gap narrows from the
pre-D8 ~6.3x to ~2.8x (composite 7.18 / async-tiled 2.54 TFLOPS): D8
makes the async-tiled kernel's ~191 single-op dispatches 5x cheaper, so
its dispatch overhead drops to ~1.5us. Qualitative order persists:
composite > async-full (3.91) > async-tiled (2.54).

test_bench_registry.py: register matmul-async / -chunked / -chunked-db
(also picks up the earlier milestone-gqa-headline -> milestone-1h-gqa
rename already present in the tree).

--- Remaining work (resume here if interrupted) ---
- Paper §3.4 (03-gemm.tex sec:gemm-vs-async): finish naive->async-full /
  chunked->async-tiled rename AND reframe "FIXED_DMA=8 for DMA descriptors"
  to single-op(8) vs composite(40). Figure already copied to
  docs/report/1H-codesign-paper/figures/gemm_composite_vs_async_tflops.png.
- Paper §3.4 K=3072 para + mechanism #3: update dispatch breakdown to new
  model (191 single-op * 8c ~= 1.5us, was 4.6us) and headline gap
  ~6.3x -> ~2.8x.
- Rebuild build/main.pdf (tectonic) + verify via pdftotext.
- Then commit Group 3 (paper + diagrams + PDF).
- Table 2 / §2 prose / ADR-0064 D8 already done in 2d8271c.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_bench_registry.py

@@ -11,10 +11,13 @@ EXPECTED_NAMES = [
     "gemm-single-pe",
     "gpt3-qkv",
     "ipcq-allreduce",
+    "matmul-async",
+    "matmul-async-chunked",
+    "matmul-async-chunked-db",
     "matmul-composite",
     "milestone-1h-ccl",
     "milestone-1h-gemm",
-    "milestone-gqa-headline",
+    "milestone-1h-gqa",
     "qkv-gemm",
     "qkv-gemm-multi-pe",
     "va-offset-verify",