feat(test): add stable param_hash key for coverage selection

Author: sbryngelson

toolchain/mfc/test/coverage.py

@@ -0,0 +1,18 @@
+"""Execution-coverage-based test selection (PR path).
+
+Selection is sound (only over-includes) and its failures are loud. See
+docs/superpowers/specs/2026-05-29-coverage-test-selection-design.md.
+"""
+
+import hashlib
+import json
+
+
+def param_hash(params: dict) -> str:
+    """Stable 16-hex key identifying a test by its defining params.
+
+    Independent of dict ordering and of the human-readable trace, so cosmetic
+    cases.py edits don't change the key; a real param change does.
+    """
+    canonical = json.dumps(params, sort_keys=True, separators=(",", ":"), default=str)
+    return hashlib.sha256(canonical.encode("utf-8")).hexdigest()[:16]

toolchain/mfc/test/test_coverage_unit.py

@@ -0,0 +1,24 @@
+from mfc.test.coverage import param_hash
+
+
+def test_param_hash_is_order_independent():
+    a = param_hash({"m": 100, "weno_order": 5, "bubbles_euler": "T"})
+    b = param_hash({"weno_order": 5, "bubbles_euler": "T", "m": 100})
+    assert a == b
+
+
+def test_param_hash_changes_with_value():
+    a = param_hash({"weno_order": 5})
+    b = param_hash({"weno_order": 3})
+    assert a != b
+
+
+def test_param_hash_is_hex_and_short():
+    h = param_hash({"m": 1})
+    assert len(h) == 16 and all(c in "0123456789abcdef" for c in h)
+
+
+def test_param_hash_nested_order_independent():
+    a = param_hash({"patch": {"x": 1, "y": 2}})
+    b = param_hash({"patch": {"y": 2, "x": 1}})
+    assert a == b