test: add pytest suite and gate Docker builds on it

Unit tests cover the SQL gate, broadcast codec, Hive writer layout,
and flight kinematics. CI runs pytest first; the simulator image build
runs tests in a dedicated stage before the final layer.

simulator/Dockerfile

@@ -1,9 +1,24 @@
+FROM python:3.12-slim AS test
+
+WORKDIR /app
+COPY requirements.txt .
+RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt pytest
+COPY virtual_drone ./virtual_drone
+COPY monitoring ./monitoring
+COPY explorer ./explorer
+COPY tests ./tests
+RUN pytest tests/ -q
+
 FROM python:3.12-slim
 
 WORKDIR /app
 COPY requirements.txt .
 RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt
 COPY virtual_drone ./virtual_drone
+# Monitoring and explorer ship in the same image so Kubernetes can run
+# them without bind mounts (Compose overrides these with live mounts)
+COPY monitoring ./monitoring
+COPY explorer ./explorer
 
 ENV DATA_DIR=/data
 CMD ["python", "-m", "virtual_drone.main"]