Check ImpedenceFinder against datasheet SI rules, not a dump table.

Each layout_rules SI requirement (diff/SE Z, skew, max length, spacing,
ref plane, vias, layer, return, series R) is one PASS/FAIL/MARGIN finding
with FACT and REQUIREMENT. USB/HDMI/PCIe/ETH/LVDS/DDR only; I2C GPIO CC
and analog REGN are skipped. No invented 90 Ω.

tests/test_layout_rules.py

@@ -18,7 +18,18 @@ def test_numeric_max_distance_mm_is_kept():
     assert ok[0]["max_distance_mm"] == given
 
 
-def test_length_match_keeps_max_distance_mm_parameter():
+def test_impedance_kind_keeps_zdiff_window():
+    ok, errors = validate_layout_rules([{
+        "kind": "impedance",
+        "zdiff_ohm": 90,
+        "tolerance_pct": 10,
+        "net_class": "usb",
+        "source_page": 12,
+    }])
+    assert errors == []
+    assert ok[0]["zdiff_ohm"] == 90
+    assert ok[0]["tolerance_pct"] == 10
+    assert ok[0]["net_class"] == "usb"
     given = 2.0
     ok, errors = validate_layout_rules([
         {"kind": "length_match", "max_distance_mm": given},