perf: replace the Plotly snap marker with a projected DOM overlay

Orbiting and the snap preview felt far behind the cursor. Measured in the
page: a single Plotly.restyle on a gl3d plot costs about 90 ms even for a
one-trace figure, so every marker update stalled the scene and the marker
trailed the mouse; each one also queued another redraw while the user was
dragging.

The marker is now a pointer-events-none div positioned by projecting the
snapped world point through glplot.cameraParams (validated: the camera
centre lands at the canvas centre), updated with one style write and
re-projected on plotly_relayout so it stays glued to the target during
orbit. No plotly calls at all on the hover path - 200 mouse-move events
now cost 0 restyles - and the empty hover trace is gone from the figure.

Also logs the WebGL renderer once (via the console bridge) since hardware
acceleration decides how smooth orbit feels and is otherwise invisible.

AGENTS.md

@@ -19,7 +19,7 @@ Strict one-way MVVM + services: `views → viewmodels → services → core`.
 - `core/` (entities: `project.py`, `geometry/`, `materials/`, `sections/`, `loads/`, `analysis/`, `catalog/`): stdlib + numpy + pydantic only. **No Qt, no openseespy. Period.**
 - `services/` (`opensees_runner.py`, `persistence.py`, `results.py`, ...): may use core + h5py + openseespy. **No Qt.**
 - `views/`: PySide6/pyvistaqt only. **No direct `import openseespy`** — go through a service.
-- `views/canvas3d/` (**PyVista/VTK**, default) and `views/canvas_plotly/` (plotly.js in a `QWebEngineView`) are two backends for the same central 3D view. Both satisfy the `CanvasBackend` protocol in `views/canvas_base.py`, share one `SelectionState` owned by `MainWindow`, and are swapped live via **Options → Canvas Backend** (persisted in `QSettings` under `canvas/backend`). `MainWindow._activate_canvas` keeps both widgets in a `QStackedWidget` — never destroy a canvas mid-session (VTK leaves dangling make-current callbacks). Backend-specific gaps are declared by `CanvasCapabilities` (e.g. Plotly has no force-diagram overlay or video export yet); gate UI on `canvas.capabilities`, never on the backend name. `canvas_plotly/trace_builder.py` is pure (no Qt, no pyvista) and unit-tested headless. `PlotlyCanvas` pushes with `Plotly.react`, which resets scene attributes the layout omits: data-only pushes pass `preserveView` so `html.py` carries the live camera and padded axis ranges forward, and the hover/snap marker is restyled only when its visible state changes (a restyle per hover event redraws per mouse move until the stack blows). Regression tests: `tests/gui/test_plotly_view_preservation.py`, `tests/gui/test_plotly_hover.py`.
+- `views/canvas3d/` (**PyVista/VTK**, default) and `views/canvas_plotly/` (plotly.js in a `QWebEngineView`) are two backends for the same central 3D view. Both satisfy the `CanvasBackend` protocol in `views/canvas_base.py`, share one `SelectionState` owned by `MainWindow`, and are swapped live via **Options → Canvas Backend** (persisted in `QSettings` under `canvas/backend`). `MainWindow._activate_canvas` keeps both widgets in a `QStackedWidget` — never destroy a canvas mid-session (VTK leaves dangling make-current callbacks). Backend-specific gaps are declared by `CanvasCapabilities` (e.g. Plotly has no force-diagram overlay or video export yet); gate UI on `canvas.capabilities`, never on the backend name. `canvas_plotly/trace_builder.py` is pure (no Qt, no pyvista) and unit-tested headless. `PlotlyCanvas` pushes with `Plotly.react`, which resets scene attributes the layout omits: data-only pushes pass `preserveView` so `html.py` carries the live camera and padded axis ranges forward, and never re-send `scene.camera`. The snap-hover marker is a `pointer-events-none` DOM overlay positioned by projecting the target through `glplot.cameraParams` — **not** a trace, because a gl3d `Plotly.restyle` costs ~90 ms per call (measured, even for one trace), which made the marker lag behind the cursor and stall orbiting. Regression tests: `tests/gui/test_plotly_view_preservation.py`, `tests/gui/test_plotly_hover.py`.
 - `views/canvas3d/style.py` (`RenderStyle`) is the single source of truth for colours/sizes on **both** backends; renderers must read it rather than hard-coding a colour. The `STYLE_FIELDS` subset is user-editable via **Options → Plot Properties…** (`views/dialogs/plot_properties.py`), previews live through `MainWindow.set_plot_style(..., persist=False)`, and persists as JSON in `QSettings` under `plot/props`.
 - `viewmodels/` bridges core↔Qt (signals, `QUndoStack`); `commands/` holds `QUndoCommand` subclasses.
 - Rules: public functions need type hints + docstring; new domain entities go through Pydantic validation; ops >50 ms run off the GUI thread (`AnalysisWorker` in QThread, cancel via `isInterruptionRequested()`, results cross threads as lightweight `ResultsHandle` to HDF5).