Rewrite DesignGraph builder as original Periscope graph.py.

Src module classifies refs, infers net type/voltage, and prefers board nets
when a .kicad_pcb is present. PinScope graph.py remains in dependency/.

tests/test_periscope_graph_rewrite.py

@@ -0,0 +1,50 @@
+"""Native graph builder lives in periscope/src; via≠pad stays on the KiCad PCB parser."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import backend.periscopex.graph as graph
+from backend.periscopex.graph import _classify_component, _infer_net_properties
+from backend.periscopex.models import ComponentType, NetType
+from backend.periscopex.parsers_kicad_pcb import LayoutPad, LayoutVia
+
+
+def test_graph_module_is_src_rewrite():
+    path = Path(graph.__file__).resolve()
+    assert path.parts[-5:] == ("periscope", "src", "backend", "periscopex", "graph.py")
+    text = path.read_text(encoding="utf-8")
+    assert "Native Periscope overlay" not in text
+    assert "original Periscope orchestration" in text
+
+
+def test_classify_prefixes_and_unknown():
+    assert _classify_component("FB1", "") == ComponentType.INDUCTOR
+    assert _classify_component("RN4", "") == ComponentType.RESISTOR
+    assert _classify_component("F1", "") == ComponentType.FUSE
+    assert _classify_component("R10", "") == ComponentType.RESISTOR
+    assert _classify_component("9", "") == ComponentType.UNKNOWN
+    assert _classify_component("9", "CONN_HEADER") == ComponentType.CONNECTOR
+
+
+def test_infer_ground_power_signal():
+    assert _infer_net_properties("GND") == (NetType.GROUND, 0.0)
+    assert _infer_net_properties("AGND") == (NetType.GROUND, 0.0)
+    ntype, volts = _infer_net_properties("+3V3")
+    assert ntype == NetType.POWER
+    assert volts == 3.3
+    ntype, volts = _infer_net_properties("3V3_DIGITAL")
+    assert ntype == NetType.POWER
+    assert volts == 3.3
+    ntype, volts = _infer_net_properties("I2C1_SCL")
+    assert ntype == NetType.SIGNAL
+    assert volts is None
+
+
+def test_via_is_not_pad():
+    via = LayoutVia(x=0.0, y=0.0, net="GND", drill=0.3)
+    pad = LayoutPad(number="1", x=0.0, y=0.0, net="GND")
+    assert type(via) is not type(pad)
+    assert not hasattr(via, "number")
+    assert pad.number == "1"
+    assert via.net == "GND"