Dedup workers: one _PooledWorker base for the three pool fan-outs

DcPrecomputeWorker, Ch4MaskWorker and CrossCorrelateWorker shared the
same submit/as_completed/emit/shutdown skeleton; they are now
constructors plus _plan/_items/_one/_emit hooks. All three inherit the
cancellation-aware shutdown (wait=not stopped, cancel_futures) that
only DcPrecomputeWorker had before, and the per-run budget attributes
are initialised in __init__ instead of appearing mid-run.

Also removes the viewer's write-only _dc_precompute_worker plumbing
(job tracking already owns worker lifetime via _jobs).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

sras_viewer.py

@@ -1478,7 +1478,6 @@ class SrasViewerWindow(QMainWindow):
         # same combination is free.
         self._dc_cache: dict[tuple[int, int], np.ndarray] = {}
         self._fft_cache: dict[tuple[int, bool, int | None, float], np.ndarray] = {}
-        self._dc_precompute_worker: DcPrecomputeWorker | None = None
         self._dc_generation: int = 0
 
         # Angle alignment ("Fusion" menu)
@@ -2463,8 +2462,7 @@ class SrasViewerWindow(QMainWindow):
         n_angles = self._sras.n_angles
 
         worker = DcPrecomputeWorker(self._sras)
-        self._dc_precompute_worker = worker
-        started = self._run_worker(
+        self._run_worker(
             "dc_precompute", worker,
             connect=(
                 ("angle_done", lambda a, dc3, dc4, g=generation:
@@ -2473,10 +2471,7 @@ class SrasViewerWindow(QMainWindow):
                     f"DC precompute error: {msg}", 5000)),
             ),
             quit_on=("finished", "error"),
-            on_done=lambda: setattr(self, "_dc_precompute_worker", None),
         )
-        if not started:
-            self._dc_precompute_worker = None
 
     def _on_dc_precompute_angle_done(self, generation: int, angle_idx: int,
                                      dc3_mv: np.ndarray, dc4_mv: np.ndarray,