Unify 2D arrows on the shared painter.

Drop orisys.util.draw_arrows; box live and replay both use
_paint_shared_flow_arrows with sdk_box_policy when no policy is passed.
Recovered box flow uses the same inverse as finger. Verified with
shared/tests/test_image_backend.py arrow/idle/recovered cases.

apps/studio/tests/test_bare_video_analysis.py

@@ -17,8 +17,8 @@ Where a failure would be invisible:
   reference frame are read back from the dialog's own widgets;
 * a recomputed force trace and a recorded one are different claims, so the charts' source
   kind and the provenance's ``sources``/``synthetic`` are asserted, not just the pixels;
-* a box result is recorded on the live painter's grid, so the canvas is compared with
-  ``draw_arrows`` on the recovered field once its gaps are filled with zero.
+* a box result is recorded on the box sample grid, so the canvas is compared with
+  ``draw_arrow_display`` on the recovered field.
 
 The fixture clip is the project's recorded box footage under ``dist/``, copied into the test's
 own directory. If it is absent the test **fails** (``_require_clip``) rather than skipping: a
@@ -433,10 +433,10 @@ def test_a_bare_video_is_analysed_and_scrubbed_in_both_directions(
 def test_the_box_arrow_canvas_is_painted_by_the_live_painter(
     bare_clip, window, monkeypatch
 ):
-    """A box pass stores the live painter's grid, so the window draws ``draw_arrows``.
+    """A box pass stores the box sample grid; the panel redraws with that policy.
 
-    The recovered field is still NaN off the recorded cells. Filling those gaps with zero
-    is what the live painter reads as "no motion", and that picture is the panel.
+    The recovered field is NaN off the recorded cells. The shared selector only samples
+    finite cells, so the panel matches ``draw_arrow_display`` on that sparse field.
     """
     open_bare_video(window, bare_clip, monkeypatch)
     settle(window)
@@ -451,7 +451,6 @@ def test_the_box_arrow_canvas_is_painted_by_the_live_painter(
         "draw_style": window._draw_style(),
         "colormap": flow_colormap(window._theme),
         "policy": policy,
-        "shared_painter": False,
     }
 
     set_mode(window, ARROW_PURE)
@@ -460,15 +459,14 @@ def test_the_box_arrow_canvas_is_painted_by_the_live_painter(
     QApplication.processEvents()
     shown = panel_bgr(window)
     assert painted(shown, canvas) > 0, "the operator's arrow canvas is blank"
-    filled = np.nan_to_num(np.asarray(first.flow, dtype=np.float64), nan=0.0)
     expected = draw_arrow_display(
         first.base_bgr,
-        filled,
+        first.flow,
         valid_mask=first.valid_mask,
         **common,
     )
     assert np.array_equal(shown, np.asarray(expected)), (
-        "the panel's canvas is not the live painter's output for this frame"
+        "the panel's canvas is not the shared painter's output for this frame"
     )