Pins: replace expected_signal_type field with a model-derived PinSpec

Introduce PinSpec (function/direction/pad/source) as the "expected" half of
pin verification, and make Pin::expected_signal_type() a derived accessor over
spec.function. pin_role() now returns a PinSpec; the connector layout (and,
later, BSDL) feed the same structure.

Pure refactor, behaviour-preserving: vpx_3u_role is still a stub, so every pin
maps to Other exactly as before. The new `pad` field will carry the BSDL
physical pin; direction/function will unlock contention/undriven/NC checks.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/system/pin_spec.hpp

@@ -0,0 +1,54 @@
+#ifndef _PIN_SPEC_HPP_
+#define _PIN_SPEC_HPP_
+
+#include "signal_type.hpp"
+
+#include <string>
+
+#pragma once
+
+// Direction of a pin as seen from the component (port/BSDL semantics).
+enum class PinDirection { Unknown, In, Out, Bidir, Passive, Power };
+
+// Functional role of a pin. Richer than SignalType (which classifies *nets*):
+// this classifies the *pin* itself and carries the TAP roles JTAG checks need.
+enum class PinFunction {
+    Unknown, Power, Ground, Signal, Clock, NoConnect,
+    JtagTdi, JtagTdo, JtagTms, JtagTck, JtagTrst
+};
+
+// Where a pin's expected attributes came from. Lets verify report conflicts and
+// lets a user override win over a model-derived value.
+enum class SpecSource { None, Imported, ConnectorModel, Bsdl, Inferred, UserOverride };
+
+// Expected, model-derived attributes of a pin (from a connector layout, a BSDL
+// model, …). The "expected" half of the expected-vs-observed verify duality;
+// the "observed" half stays Pin::signal() + the net + inference.
+struct PinSpec {
+    PinFunction  function  = PinFunction::Unknown;
+    PinDirection direction = PinDirection::Unknown;
+    std::string  pad;                    ///< Physical terminal (package ball/pin); "" if unknown.
+    SpecSource   source    = SpecSource::None;
+};
+
+// PinFunction -> the net SignalType a pin of that function expects to sit on.
+inline SignalType to_signal_type(PinFunction f)
+{
+    switch (f) {
+    case PinFunction::Power:  return SignalType::Power;
+    case PinFunction::Ground: return SignalType::GndShield;
+    default:                  return SignalType::Other;
+    }
+}
+
+// Coarse SignalType -> PinFunction, used when a model only yields a SignalType.
+inline PinFunction function_from_signal_type(SignalType t)
+{
+    switch (t) {
+    case SignalType::Power:     return PinFunction::Power;
+    case SignalType::GndShield: return PinFunction::Ground;
+    default:                    return PinFunction::Unknown;
+    }
+}
+
+#endif // _PIN_SPEC_HPP_