fix(sim): trace through BJT C↔B in getArduinoPinHelper

The canonical "Arduino pin → resistor → BJT base, BJT collector →
load" pattern for multiplexed 7-segment clocks was breaking in the
simulator: getArduinoPinHelper('COM.1') couldn't resolve through
the transistor, so the multiplex-aware 7-segment driver thought no
digit-select pin was wired and fell back to "all digits enabled".
Result: every display in the multiplex array rendered the same
rapidly-changing pattern → user-visible flicker.

Fix: add the NPN/PNP BJTs to the PASSIVE_PIN_PAIRS map with
[collector, base] — the trace function continues from B when it
arrives at C (and vice versa). That makes the Arduino pin driving
the base reported as the controller of the collector — exactly the
relationship the user's multiplex code expects.

Conventions covered:
  - NPN (2n2222, bc547, 2n3055): Arduino HIGH → transistor on →
    COM pulled LOW → common-cathode digit enabled.  Our 7-segment
    driver treats "digit pin HIGH = enabled" which matches.
  - PNP (2n3906, bc557): inverse logic.  We expose the same pin
    mapping; users writing PNP-driver code will see the polarity
    behave inverted, which is what real hardware does too.

This is a one-line shortcut, not a true active-device model. We're
not simulating BJT saturation, β, base current, or PNP polarity —
just reporting "this Arduino pin is the boss of this collector".
That's enough for the multiplexing use case and the only place
getArduinoPinHelper is consulted today.

frontend/src/components/DynamicComponent.tsx

@@ -262,6 +262,24 @@ export const DynamicComponent: React.FC<DynamicComponentProps> = ({
           // NTC and photoresistor breakouts are 3-pin active modules (VCC/GND
           // + analog output); not traceable as 2-terminal passives. Their
           // analog output is already an ADC-readable pin on its own.
+          //
+          // BJTs are 3-pin actives, but the canonical "Arduino digital pin
+          // controls a load via transistor" pattern is fundamental enough
+          // that we treat them as a [collector, base] shortcut.  Tracing
+          // FROM the collector side continues through the base — i.e. the
+          // Arduino pin driving the base is reported as the controller of
+          // the collector. That makes 7-segment multiplex circuits with
+          // BJT digit drivers actually work in the simulator, since
+          // getArduinoPinHelper('COM.1') can resolve through the transistor.
+          // For NPN, Arduino HIGH at base → transistor on → collector pulled
+          // to emitter (typically GND) — and "HIGH = digit enabled" in our
+          // 7-segment driver matches this when COM is common-cathode wired
+          // through the transistor to GND.
+          'bjt-2n2222': ['C', 'B'],
+          'bjt-bc547': ['C', 'B'],
+          'bjt-2n3055': ['C', 'B'],
+          'bjt-2n3906': ['C', 'B'],
+          'bjt-bc557': ['C', 'B'],
         };
         // Preset variants of the generic passives share their parent's tag
         // and pin layout — so resistor-220, cap-1u, ind-10m, etc. trace the