ci: add ESP32 + ngspice co-simulation tests to backend E2E pipeline

Adds three new test steps to backend-e2e-tests.yml:
- ngspice smoke test (standalone, no backend needed)
- ESP32 voltage divider co-simulation (compile + QEMU + ngspice ADC injection)
- ESP32 Wheatstone bridge co-simulation (NTC temperature sweep)

These run alongside the existing DHT22/HC-SR04/MPU6050/MicroPython tests
using the same QEMU .so libraries and ESP32 Arduino core 2.0.17.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/backend-e2e-tests.yml

@@ -168,6 +168,20 @@ jobs:
         env:
           BACKEND_URL: http://localhost:8001
 
+      # ── ESP32 + ngspice co-simulation tests ─────────────────────────────────
+      - name: Run ngspice smoke test (no backend needed)
+        run: node test/backend/e2e/test_esp32_spice_smoke.mjs
+
+      - name: Run ESP32 + ngspice voltage divider co-simulation
+        run: node test/backend/e2e/test_esp32_spice_analog.mjs --timeout=120
+        env:
+          BACKEND_URL: http://localhost:8001
+
+      - name: Run ESP32 + ngspice Wheatstone bridge co-simulation
+        run: node test/backend/e2e/test_esp32_spice_ntc_bridge.mjs --timeout=120
+        env:
+          BACKEND_URL: http://localhost:8001
+
       # ── Always dump backend logs so failures are diagnosable ──────────────────
       - name: Show backend logs
         if: always()