fix(benchmarks): pin deterministic pytest-randomly seed for host shards (#625)

Benchmark host-validation shards ran without a pinned pytest-randomly
seed, so each CI runner used a different random collection order.
pytest-split's LeastDurationAlgorithm partitions the differently ordered
collections independently, assigning the same test to multiple groups
and producing 58 duplicate timing entries across four shards.

Wire the existing pytest_randomly_shard_seed from .github/ci-config.json
(the same SSOT used by the CI workflow for domain/composition shards)
through pytest_arguments() in host_validation.py, injecting --randomly-seed
into every shard's pytest command vector. This ensures all shards see
an identical collection order, so pytest-split produces disjoint groups.

The defensive merge behavior from #624 remains intact as a second line
of defense.

Generated with [Devin](https://devin.ai)

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: morluto

benchmarks/tooling/host_validation.py

@@ -25,6 +25,7 @@
 
 ROOT = Path(__file__).resolve().parents[2]
 TIMING_PATH = ROOT / ".ci" / "benchmark-test-durations.json"
+_CI_CONFIG = ROOT / ".github" / "ci-config.json"
 _DIGEST = re.compile(r"sha256:[0-9a-f]{64}\Z")
 _SHA = re.compile(r"[0-9a-f]{40,64}\Z")
 _HOST_NAME = re.compile(r"[A-Za-z0-9_.-]+\Z")
@@ -33,6 +34,16 @@
 _MAX_TIMING_ENTRIES = 10_000
 
 
+def _shard_seed() -> int:
+    """Return the pinned pytest-randomly seed from ci-config.json."""
+
+    payload = json.loads(_CI_CONFIG.read_text(encoding="utf-8"))
+    seed = payload.get("pytest_randomly_shard_seed")
+    if not isinstance(seed, int) or isinstance(seed, bool) or seed < 0:
+        raise ValueError("pytest_randomly_shard_seed must be a nonnegative integer")
+    return seed
+
+
 @dataclass(frozen=True, slots=True)
 class ExecutionProvenance:
     """Immutable identities shared by every shard in one host-validation run."""
@@ -295,6 +306,7 @@ def pytest_arguments(
     arguments = ["-n", str(workers), "--durations=10", entry.selector]
     if entry.keyword:
         arguments.extend(("-k", entry.keyword))
+    arguments.extend(("--randomly-seed", str(_shard_seed())))
     if entry.splits:
         arguments.extend(
             (

benchmarks/validation/test_host_validation.py

@@ -75,6 +75,48 @@ def test_full_shard_uses_ci_timing_shape_and_least_duration(tmp_path: Path) -> N
     assert "--store-durations" not in arguments
 
 
+def test_all_shards_receive_same_deterministic_seed(tmp_path: Path) -> None:
+    """Every full-host shard must get the same pytest-randomly seed so that
+    pytest-split sees an identical collection order across CI runners."""
+
+    entries = full_host_validation()
+    seeds: set[str] = set()
+    for entry in entries:
+        arguments = pytest_arguments(
+            entry,
+            timing_path=tmp_path / "timings.json",
+            workers=2,
+            store_durations=False,
+        )
+        seed_index = arguments.index("--randomly-seed")
+        seeds.add(arguments[seed_index + 1])
+    assert len(seeds) == 1
+    assert seeds.pop() == "0"
+
+
+def test_keyword_filtered_entry_also_receives_deterministic_seed(
+    tmp_path: Path,
+) -> None:
+    from benchmarks.tooling.validation_plan import task_host_validation
+
+    entries = task_host_validation(
+        Path(__file__).resolve().parents[2],
+        "mathematical-benchmarks-v1",
+        "algebraic-independence-transfer-audit",
+    )
+    assert entries
+    for entry in entries:
+        arguments = pytest_arguments(
+            entry,
+            timing_path=tmp_path / "timings.json",
+            workers=1,
+            store_durations=False,
+        )
+        assert "--randomly-seed" in arguments
+        seed_index = arguments.index("--randomly-seed")
+        assert arguments[seed_index + 1] == "0"
+
+
 def test_local_full_run_uses_empty_timing_fallback_and_writes_bound_receipts(
     tmp_path: Path,
 ) -> None: