tech-debt: reinstate example smoke-test coverage, and cover examples/codegen/ separately

Summary

tests/test_examples_smoke.py (runs every top-level examples/*.py script as a subprocess, asserts exit 0) is currently fully disabled: pytestmark = pytest.mark.skip(reason="example smoke tests temporarily disabled"), no further explanation in the file. Worth understanding why and reinstating if the original reason no longer applies.

examples/codegen/ is deliberately out of its scope, not an oversight

bdsim.codegen's example scripts (motor_control.py, verify_codegen.py, main.cpp) live in examples/codegen/, a subdirectory. test_examples_smoke.py globs non-recursively (EXAMPLES_DIR.glob("*.py")), so they're excluded from its discovery automatically -- intentional, not something to "fix" by making the glob recursive. They need a C++ toolchain (clang++ + Eigen) that a generic bdsim smoke test shouldn't have to assume is present; tests/test_codegen.py covers codegen-specific compile-verification separately, with its own skipif guard for when a compiler/Eigen aren't available.

When reinstating this test, keep examples/codegen/ excluded (either leave the glob non-recursive, or add an explicit skip for that subdirectory if the glob is ever made recursive for other reasons).

Start with tests/test_examples_smoke.py and its disabled pytestmark, then inspect tests/test_codegen.py and the examples/ layout to understand the existing coverage and environment guards. Reinstate the top-level example subprocess smoke test if its original blocker no longer applies, while keeping examples/codegen/ excluded; verify with the relevant pytest tests.