verify: model-driven pin checks (contention / undriven / NC-wired)

New bsdl_check.{hpp,cpp}: check_pin_specs(System*) walks the nets and uses
each pin's PinSpec direction/function to flag DriveContention (>=2 push-pull
output drivers), UndrivenNet (a multi-pin net with input(s) but no driver),
and NcWired (a no-connect pin wired onto a multi-pin net). Added as a pass in
`verify`; AnomalyKind extended accordingly. Nets with no direction data are
skipped, so un-modelled parts produce no noise. Covered by test_bsdl_check.

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

src/system/bsdl_check.cpp

@@ -0,0 +1,111 @@
+#include "bsdl_check.hpp"
+
+#include "modules.hpp"
+#include "nets.hpp"
+#include "parts.hpp"
+#include "pin_spec.hpp"
+#include "pins.hpp"
+#include "signals.hpp"
+#include "system.hpp"
+
+#include <string>
+#include <vector>
+
+namespace {
+
+std::string pin_label(const Pin *p)
+{
+    std::string part = (p && p->prnt) ? p->prnt->name : std::string("?");
+    return part + "/" + (p ? p->name : std::string("?"));
+}
+
+std::string join_labels(const std::vector<Pin *> &pins)
+{
+    std::string s;
+    for (const Pin *p : pins)
+        s += (s.empty() ? "" : ", ") + pin_label(p);
+    return s;
+}
+
+}  // namespace
+
+std::vector<Anomaly> check_pin_specs(System *sys)
+{
+    std::vector<Anomaly> out;
+    if (!sys)
+        return out;
+
+    for (const Net &net : compute_all_nets(sys)) {
+        std::vector<Pin *> pins;
+        std::vector<Signal *> sigs;
+        Module *mod = nullptr;
+        std::string label;
+
+        for (const auto &mp : net.members) {
+            if (!mod)
+                mod = mp.first;
+            sigs.push_back(mp.second);
+            if (label.empty() && mp.first && mp.second)
+                label = mp.first->name + "/" + mp.second->name;
+            for (auto &kv : *mp.second)
+                pins.push_back(kv.second);
+        }
+
+        if (pins.size() < 2)
+            continue;  // singleton net — nothing to compare
+
+        int outs = 0, drivers = 0, ins = 0, known = 0;
+        std::vector<Pin *> out_pins, in_pins, nc_pins;
+
+        for (Pin *p : pins) {
+            const PinSpec &s = p->spec;
+            if (s.function == PinFunction::NoConnect)
+                nc_pins.push_back(p);
+            if (s.direction == PinDirection::Unknown)
+                continue;
+            ++known;
+            switch (s.direction) {
+            case PinDirection::Out:   ++outs; ++drivers; out_pins.push_back(p); break;
+            case PinDirection::Bidir: ++drivers; break;
+            case PinDirection::Power: ++drivers; break;
+            case PinDirection::In:    ++ins; in_pins.push_back(p); break;
+            default: break;
+            }
+        }
+
+        // A no-connect pin that is nonetheless on a multi-pin net.
+        for (Pin *p : nc_pins) {
+            Anomaly a;
+            a.kind = AnomalyKind::NcWired;
+            a.module = mod;
+            a.involved = sigs;
+            a.message = pin_label(p) + " is marked no-connect but wired to net " + label;
+            out.push_back(std::move(a));
+        }
+
+        if (!known)
+            continue;  // no direction data on this net — skip drive checks
+
+        if (outs >= 2) {
+            Anomaly a;
+            a.kind = AnomalyKind::DriveContention;
+            a.module = mod;
+            a.involved = sigs;
+            a.message = "net " + label + ": " + std::to_string(outs)
+                      + " output drivers (" + join_labels(out_pins) + ")";
+            out.push_back(std::move(a));
+        }
+
+        if (ins >= 1 && drivers == 0) {
+            Anomaly a;
+            a.kind = AnomalyKind::UndrivenNet;
+            a.module = mod;
+            a.involved = sigs;
+            a.message = "net " + label + ": input(s) with no driver ("
+                      + join_labels(in_pins) + ")";
+            out.push_back(std::move(a));
+        }
+    }
+
+    return out;
+}