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/tui/screen_dashboard.cpp

@@ -83,7 +83,7 @@ Component Tui::BuildDashboardScreen() {
               for (auto &nkv : *prt) {
                   Pin *pin = nkv.second;
                   ++n_typed_pins;
-                  SignalType expected = pin->expected_signal_type;
+                  SignalType expected = pin->expected_signal_type();
                   if (expected == SignalType::Other) continue;
                   Signal *s = pin->signal();
                   SignalType actual = s ? s->type : SignalType::Other;