fix: stop the Plotly hover handler redrawing on every mouse move

plotly_hover/plotly_unhover fire continuously while the mouse moves, and
the unhover handler restyled the snap marker unconditionally - even with
snapping off and nothing visible. That meant a full Plotly.restyle plus
redraw per mouse event, which re-fired hover until the page died with
'RangeError: Maximum call stack size exceeded' and the canvas froze
(reported live after opening a model).

The marker is now updated only when its visible state actually changes: no
marker when snapping is off, and a restyle only when the snapped point
differs from the one already shown. Deliberately synchronous - timers are
throttled to about a second by WebEngine when the page is not compositing,
which stalled the preview. Measured: 200 mouse-move events went from 200
restyles to 0; 200 hovers on one grid dot cost a single restyle.

tests/gui/test_plotly_hover.py

@@ -0,0 +1,93 @@
+"""Regression tests for the Plotly hover/snap-marker contract.
+
+``plotly_hover``/``plotly_unhover`` fire continuously while the mouse moves.
+Touching the plot on each one made it redraw per mouse move, re-firing hover
+until the stack blew (``RangeError: Maximum call stack size exceeded``) and the
+canvas froze. The marker is therefore coalesced and only touched when its
+shown/hidden state actually changes.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+pytest.importorskip("PySide6")
+pytest.importorskip("plotly")
+
+from otko.services import load_project
+
+EXAMPLES = Path(__file__).resolve().parents[2] / "examples"
+
+#: 200 mouse-move events over a *non-snap* point with snapping armed.
+_CHURN_PROBE = """(function(){
+  var gd = document.getElementById('plot');
+  var n = 0; var original = Plotly.restyle;
+  Plotly.restyle = function () { n++; return original.apply(Plotly, arguments); };
+  var point = { data: gd.data[0], customdata: [0, 0, 0] };
+  try {
+    for (var i = 0; i < 200; i++) {
+      gd.emit('plotly_hover', { points: [point] });
+      gd.emit('plotly_unhover', {});
+    }
+    return n;
+  } finally { Plotly.restyle = original; }
+})()"""
+
+_HOVER_MARKER_X = (
+    "JSON.stringify((document.getElementById('plot').data"
+    ".find(function(d){return d.meta && d.meta.kind === 'hover';}) || {}).x)"
+)
+
+_EMIT_SNAP_HOVER = """(function(){
+  var gd = document.getElementById('plot');
+  var snap = gd.data.filter(function(d){return d.meta && d.meta.kind === 'snap';})[0];
+  var point = { data: snap, customdata: [snap.x[0], snap.y[0], snap.z[0]] };
+  gd.emit('plotly_hover', { points: [point] });
+  return true;
+})()"""
+
+
+def _open_canvas(qtbot, name: str):  # type: ignore[no-untyped-def]
+    from otko.views.canvas_plotly import PlotlyCanvas
+
+    canvas = PlotlyCanvas()
+    qtbot.addWidget(canvas)
+    canvas.show_project(load_project(EXAMPLES / name))
+    qtbot.waitUntil(lambda: canvas._ready, timeout=30000)
+    qtbot.wait(1500)  # let the first Plotly.react settle
+    return canvas
+
+
+def _run_js(canvas, qtbot, script: str, timeout: int = 15000):  # type: ignore[no-untyped-def]
+    box: dict[str, object] = {}
+    canvas._web.page().runJavaScript(script, lambda value: box.update(value=value))
+    qtbot.waitUntil(lambda: "value" in box, timeout=timeout)
+    return box["value"]
+
+
+@pytest.mark.gui
+def test_ordinary_mouse_movement_does_not_redraw(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """The unfixed version restyled once per hover/unhover event."""
+    canvas = _open_canvas(qtbot, "space_frame_3d.osmodel")
+
+    restyles = _run_js(canvas, qtbot, _CHURN_PROBE)
+
+    assert restyles == 0, f"mouse movement caused {restyles} plot restyles"
+
+
+@pytest.mark.gui
+def test_snap_marker_shows_and_clears(qtbot) -> None:  # type: ignore[no-untyped-def]
+    canvas = _open_canvas(qtbot, "basic_truss.osmodel")
+    assert _run_js(canvas, qtbot, _HOVER_MARKER_X) == "[]"
+
+    canvas.set_snap_preview_enabled(True)
+    qtbot.wait(100)
+    _run_js(canvas, qtbot, _EMIT_SNAP_HOVER)
+    qtbot.wait(250)
+    assert _run_js(canvas, qtbot, _HOVER_MARKER_X) != "[]", "snap target not shown"
+
+    _run_js(canvas, qtbot, "document.getElementById('plot').emit('plotly_unhover',{})")
+    qtbot.wait(250)
+    assert _run_js(canvas, qtbot, _HOVER_MARKER_X) == "[]", "snap target not cleared"