Reduce test time to 12s: shrink GEMM dims + enable pytest-xdist

GEMM dimension reduction:
- qkv_gemm.py: M,K,N = 128,256,128 → 32,64,32 (64 tiles → 1 tile).
- qkv_gemm_multi_pe.py: same reduction.
- Tests verify pipeline correctness, not large-matrix throughput.
- Per-test time: 18s → 1.7s. 6 tests total: 108s → 10s.

pytest-xdist parallel execution:
- Add pytest-xdist to dev dependencies.
- pyproject.toml addopts: -n auto (use all CPU cores), -m "not slow".
- Default `pytest` runs 501 tests in ~12s (previously 148s).
- Full suite including slow: `pytest -m ""` → 3m24s (previously 5m43s).

pytest.mark.slow:
- Registered in pyproject.toml markers section.
- 256-rank full-system test is the only slow-marked test.
- Run with: pytest -m "" (CI) or pytest (local dev, skips slow).

502 tests pass.

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

benches/qkv_gemm.py

@@ -10,7 +10,9 @@ Kernel: tl.load(a) + tl.ref(b) + tl.composite(gemm) + tl.wait()
 from kernbench.policy.placement.dp import DPPolicy
 
 # GEMM dimensions: (M, K) x (K, N) → (M, N)
-M, K, N = 128, 256, 128
+# Small dims (1 tile) for fast regression. The test verifies the full
+# host→PE pipeline, not large-matrix throughput.
+M, K, N = 32, 64, 32
 DTYPE = "f16"
 
 

benches/qkv_gemm_multi_pe.py

@@ -10,7 +10,9 @@ Kernel: tl.load(a) + tl.ref(b) + tl.composite(gemm) + tl.wait()
 from kernbench.policy.placement.dp import DPPolicy
 
 # GEMM dimensions: (M, K) x (K, N) -> (M, N)
-M, K, N = 128, 256, 128
+# Small dims (1 tile) for fast regression. The test verifies the multi-PE
+# fan-out pipeline, not large-matrix throughput.
+M, K, N = 32, 64, 32
 DTYPE = "f16"