replace eris-worker subprocess with ProcessPoolExecutor; guard sweep generators against empty _entries

- Rewrite worker.py: each Worker uses a ProcessPoolExecutor(max_workers=1),
  tools run via loop.run_in_executor() in a forked OS subprocess
- Remove create_process(), main(), stdin/stdout IPC, os.killpg
- Remove eris-worker entry point from [project.scripts] in pyproject.toml
- Update tests/worker_test.py for new Worker API
- Guard _sweep_up/_sweep_down against IndexError when _entries is empty;
  keep cursor_position() try/except for stale y after row deletion

tests/worker_test.py

@@ -19,16 +19,16 @@ class WorkerTestCase(unittest.TestCase):
         open("foo", "w").close()
 
     def tearDown(self):
+        if hasattr(self, "worker_"):
+            self.worker_.kill()
         shutil.rmtree(self.temp_dir)
         os.chdir(self.original_working_dir)
 
     def test_run_job(self):
         loop = asyncio.new_event_loop()
         compression = "none"
-        worker_ = worker.Worker(compression)
-        loop.run_until_complete(worker_.create_process())
-        worker_.process.stdin.write(f"{compression}\n".encode("utf-8"))
-        future = worker_.run_tool("foo", tools.metadata)
+        self.worker_ = worker.Worker(compression)
+        future = self.worker_.run_tool("foo", tools.metadata)
         status = loop.run_until_complete(future)
         self.assertEqual(status, tools.Status.ok)
         result_path = os.path.join(tools.CACHE_PATH, "foo-metadata")