The esp32_worker.py subprocess is launched via `python <abs_path>`
and runs with a sys.path that does NOT include the backend/ package
root, so `from app.services.signal_router import SignalRouter`
raised ModuleNotFoundError at worker startup. The worker exited
with code 1 before QEMU even loaded, and the frontend surfaced the
generic "ESP32 crash detected — cache error" banner.
Mirror the existing esp32_flash_image fallback pattern (already in
this same file): try the package import first, fall back to
importlib.spec_from_file_location with the sibling .py path, then
publish the resulting module under its bare name in sys.modules so
typing references continue to work.
Verified: a synthetic test that strips backend/ from sys.path can
still construct a SignalRouter via the fallback. 20 unit tests in
test_signal_router.py still pass.
f6131d432e