Rewrite Periscope models.py with disjoint layout primitives.

Pad, via, track, and zone stay separate types. Inherited models.py remains
in dependency/. Public graph/finding fields are unchanged.

tests/test_periscope_models_rewrite.py

@@ -0,0 +1,44 @@
+"""Periscope models load from src; layout primitives stay disjoint."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import backend.periscopex.models as models
+from backend.periscopex.models import (
+    LayoutPad,
+    LayoutSegment,
+    LayoutVia,
+    LayoutZone,
+)
+
+
+def test_models_module_is_src():
+    path = Path(models.__file__).resolve()
+    assert path.parts[-5:] == ("periscope", "src", "backend", "periscopex", "models.py")
+    text = path.read_text(encoding="utf-8")
+    assert "Native Periscope overlay" not in text
+    assert "pad (component pin land)" in text
+
+
+def test_pad_via_track_zone_are_distinct_types():
+    pad = LayoutPad(number="1", x=0.0, y=0.0, net="GND")
+    via = LayoutVia(x=0.0, y=0.0, net="GND", drill=0.3)
+    track = LayoutSegment(start=(0.0, 0.0), end=(1.0, 0.0), net="GND")
+    zone = LayoutZone(net="GND", layer="F.Cu")
+    kinds = {type(pad), type(via), type(track), type(zone)}
+    assert len(kinds) == 4
+    assert not hasattr(via, "number")
+    assert hasattr(pad, "number")
+    assert not hasattr(track, "drill")
+    assert not hasattr(zone, "drill")
+    assert pad.model_dump() != via.model_dump()
+
+
+def test_via_is_not_a_pad_even_on_same_net_and_xy():
+    """Negative: same net and coordinates do not make a via a pad."""
+    pad = LayoutPad(number="2", x=1.0, y=2.0, net="3V3")
+    via = LayoutVia(x=1.0, y=2.0, net="3V3", drill=0.2)
+    assert type(via) is not type(pad)
+    assert "number" not in type(via).model_fields
+    assert "number" in type(pad).model_fields