fix(s31): close 1 review MAJOR — VECLIB_MAXIMUM_THREADS + honest determinism claim + the missing pin guard

tests/test_semretrieval_loadbearing.py

@@ -26,6 +26,7 @@ system's own echo of the query can no longer outrank genuine expert prose.
 from __future__ import annotations
 
 import ast
+import os
 import subprocess
 import sys
 from pathlib import Path
@@ -106,6 +107,54 @@ def test_semretrieval_import_and_rank_pull_in_no_maf_and_no_verdicts() -> None:
     assert result.returncode == 0, result.stderr
 
 
+def test_blas_thread_pins_are_set_before_numpy_is_imported() -> None:
+    """SC3 GUARD — the determinism claim's one untested half, until now.
+
+    Two properties, both checked in a FRESH subprocess that execs the module standalone:
+    1. all four thread variables are set to ``"1"`` after the module body runs;
+    2. ``numpy`` is absent from ``sys.modules`` BEFORE the module's env writes execute — the
+       ordering the whole mechanism depends on, since every backend latches its variable by the
+       time the numpy import completes.
+
+    ``VECLIB_MAXIMUM_THREADS`` is the one that BITES on this build (numpy links Accelerate); the
+    other three cover OpenBLAS/MKL/OpenMP deployments. Before it was added this block pinned
+    nothing at all here, and no test could tell — ``grep -rn 'OPENBLAS\\|OMP_NUM\\|MKL_NUM' tests/``
+    returned zero hits.
+
+    Detach point: move any ``os.environ.setdefault`` below the ``import numpy`` line → RED."""
+    check = (
+        "import importlib.util, sys\n"
+        "assert 'numpy' not in sys.modules, 'numpy was imported before the probe started'\n"
+        f"spec = importlib.util.spec_from_file_location('semretrieval_pins', {str(_SEMRETRIEVAL)!r})\n"
+        "mod = importlib.util.module_from_spec(spec)\n"
+        "\n"
+        "# Trip-wire: record whether numpy was already imported at the moment the env writes ran.\n"
+        "import os\n"
+        "seen = {}\n"
+        "real_setdefault = os.environ.setdefault\n"
+        "def spy(key, value):\n"
+        "    seen.setdefault(key, 'numpy' in sys.modules)\n"
+        "    return real_setdefault(key, value)\n"
+        "os.environ.setdefault = spy\n"
+        "try:\n"
+        "    spec.loader.exec_module(mod)\n"
+        "finally:\n"
+        "    os.environ.setdefault = real_setdefault\n"
+        "\n"
+        "for var in ('VECLIB_MAXIMUM_THREADS', 'OPENBLAS_NUM_THREADS', 'OMP_NUM_THREADS',\n"
+        "            'MKL_NUM_THREADS'):\n"
+        "    assert os.environ.get(var) == '1', var + ' is ' + repr(os.environ.get(var))\n"
+        "    assert var in seen, var + ' was never pinned by the module body'\n"
+        "    assert seen[var] is False, var + ' was pinned AFTER numpy was imported'\n"
+        "assert 'numpy' in sys.modules, 'the module never imported numpy — probe is not exercising it'\n"
+    )
+    env = {
+        k: v for k, v in os.environ.items() if not k.endswith(("_NUM_THREADS", "_MAXIMUM_THREADS"))
+    }
+    result = subprocess.run([sys.executable, "-c", check], capture_output=True, text=True, env=env)
+    assert result.returncode == 0, result.stderr
+
+
 def test_semretrieval_imports_no_network_modules() -> None:
     """SC8 — the offline default must not be able to reach the network. Detach point: add
     ``import urllib.request`` (or any ``_NETWORK_ROOTS`` member) to semretrieval.py → RED."""