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.hpp

@@ -0,0 +1,19 @@
+#ifndef _BSDL_CHECK_HPP_
+#define _BSDL_CHECK_HPP_
+
+#include "analysis.hpp"  // Anomaly, AnomalyKind
+
+#include <vector>
+
+class System;
+
+// Model-driven pin checks over the system's nets, using the PinSpec
+// direction/function populated by connector or BSDL models. Emits:
+//   - DriveContention : a net with ≥2 push-pull output drivers;
+//   - UndrivenNet     : a multi-pin net with input(s) but no driver;
+//   - NcWired         : a no-connect pin wired onto a multi-pin net.
+// Read-only; nets with no direction data are skipped (no false positives on
+// un-modelled parts).
+std::vector<Anomaly> check_pin_specs(System *sys);
+
+#endif // _BSDL_CHECK_HPP_