Pin fork start method in frontend for Python 3.14

Python 3.14 changed the multiprocessing default from fork to forkserver
on Linux, which pickles the target process. The Frontend's service
instances carry pipes and transport state that aren't pickleable, so
the previous reliance on fork semantics no longer works out of the box.

Force the fork start method at module load to preserve existing behaviour.
fork is deprecated upstream and will eventually need a real refactor.

Author: ndevenish

src/workflows/frontend/__init__.py

@@ -16,6 +16,12 @@
 
 basestring = (str, bytes)
 
+# Pin the fork start method: service instances carry pipes and transport
+# state that aren't pickleable, so spawn/forkserver (the 3.14+ default on
+# Linux) can't serialize them. fork is deprecated upstream and will
+# eventually need a real refactor.
+multiprocessing.set_start_method("fork", force=True)
+
 
 class Frontend:
     """The frontend class encapsulates the actual service. It controls the