test(s31): close 1 review MAJOR — guard the sole-numpy-importer premise the BLAS pin rests on

tests/test_semretrieval_loadbearing.py

@@ -167,6 +167,46 @@ def test_blas_thread_pins_are_set_before_numpy_is_imported() -> None:
     assert result.returncode == 0, result.stderr
 
 
+def test_semretrieval_is_the_sole_numpy_importer() -> None:
+    """The PREMISE the pin above rests on, guarded — it was prose until now.
+
+    ``semretrieval.py`` can pin the BLAS thread variables at import time only because it is the
+    first and only place numpy enters the package. A second importer elsewhere under ``src/``
+    would very likely be imported FIRST (``run.py``, ``verdicts.py`` and friends load long before
+    the opt-in retrieval seam), numpy would latch its backend's thread count before these
+    ``setdefault`` calls ever run, and the pin would become a silent no-op in the real process —
+    while ``test_blas_thread_pins_are_set_before_numpy_is_imported`` stayed green, because that
+    probe execs THIS module standalone in a fresh interpreter and can never observe the collision.
+
+    AST, not substring: a docstring mentioning numpy must not trip the guard, exactly as
+    ``test_okf_is_maf_free`` reasons about the MAF-free claim.
+
+    Deliberate limit, stated rather than oversold: this covers ``src/`` code, which is what the
+    premise actually claims. A third-party dependency importing numpy transitively is out of
+    scope — the pins are defence-in-depth for byte-identical artifacts, never the basis of the
+    ranking guarantee (see the module docstring).
+
+    Detach point: add ``import numpy`` to any other module under ``src/portfolio_optimiser/``
+    → RED, while the rest of the suite stays green."""
+    package = Path(__file__).resolve().parents[1] / "src" / "portfolio_optimiser"
+    importers: set[str] = set()
+    for path in sorted(package.glob("*.py")):
+        roots: set[str] = set()
+        for node in ast.walk(ast.parse(path.read_text(encoding="utf-8"))):
+            if isinstance(node, ast.Import):
+                roots.update(alias.name.split(".")[0] for alias in node.names)
+            elif isinstance(node, ast.ImportFrom) and node.module:
+                roots.add(node.module.split(".")[0])
+        if "numpy" in roots:
+            importers.add(path.name)
+
+    assert importers == {"semretrieval.py"}, (
+        f"numpy is imported by {sorted(importers)}; the import-time BLAS pin in semretrieval.py "
+        "is only viable while that module is the SOLE numpy importer. Another importer loaded "
+        "earlier makes the pin a no-op in the real process and no existing test can see it."
+    )
+
+
 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."""