revert worker to subprocess model, self-spawning via --worker flag

ProcessPoolExecutor relied on private API, lost os.setpriority and
auto-restart. Revert to the subprocess worker but spawn the current
executable with --worker instead of a separate eris-worker console
script, keeping a single entry point for shiv/pyinstaller. Bump
.python-version to 3.14.

tests/worker_test.py

@@ -2,6 +2,7 @@
 import asyncio
 import os
 import shutil
+import sys
 import tempfile
 import unittest
 
@@ -28,6 +29,8 @@ class WorkerTestCase(unittest.TestCase):
         loop = asyncio.new_event_loop()
         compression = "none"
         self.worker_ = worker.Worker(compression)
+        command = [sys.executable, "-m", "eris.worker", "--worker"]
+        loop.run_until_complete(self.worker_.create_process(command=command))
         future = self.worker_.run_tool("foo", tools.metadata)
         status = loop.run_until_complete(future)
         self.assertEqual(status, tools.Status.ok)