feat: introduce family_spec logic for wafer sensor-count partitioning

src/pygui/backend/wafer/family_spec.py

@@ -0,0 +1,9 @@
+"""Single source of truth for the wafer sensor-count partition."""
+
+
+def sensor_count_for(family_code: str) -> int:
+    """Return the number of valid sensor readings for a wafer family code.
+
+    Returns 80 for the "X" family. Returns 244 for every other family
+    code."""
+    return 80 if family_code == "X" else 244