Add clipping planes, axes inset, and named click intersections to Scene

[Jepson2k]

Motivation

Three related Scene additions that come up constantly in 3D-visualization apps and round out the interactivity story:

• Clipping planes — hide parts of a mesh against arbitrary plane equations to "see inside" articulated geometry.
• Orientation axes inset — small XYZ gizmo in the corner that follows the camera, so the user always knows which way is "up". Click an axis to snap-animate the camera along it.
• Named click intersections — declare named planes once, then read each click ray's intersection with each plane on every click event. Generalizes the common "click ground to place" workflow without hard-coding a single ground plane.

Plus raycaster_threshold becomes a real watched prop instead of an init-only value.

This is one of several small slices being carved out of #5964 per the splitting discussion on that thread.

Implementation

Clipping planes (nicegui/elements/scene/scene_object3d.py, nicegui/elements/scene/scene.js, nicegui/events.py):

• New SceneClipPlane(nx, ny, nz, d) dataclass.
• Object3D.set_clipping_planes(planes: list[SceneClipPlane]) and clear_clipping_planes() chained methods.
• JS-side: builds a THREE.Plane per spec, applies as material.clippingPlanes to every mesh descendant, and flips the renderer's localClippingEnabled flag on first use. clipIntersection = false means clipping is the union across planes.

Axes inset (Scene.set_axes_inset, Scene.set_axes_labels):

• Typed kwargs (no untyped dict[str, Any]):

  Scene.set_axes_inset(*, enabled=True, size=128, margin=0,
                       margin_x=None, margin_y=None,
                       anchor='bottom-right')  # -> Self
  Scene.set_axes_labels(*, enabled=True, labels=('X', 'Y', 'Z'),
                        font='24px Arial', color='#000000', radius=14)  # -> Self

• JS-side: lazily constructs Three.js' ViewHelper, then composites it into the configured corner inside the existing render loop using manual scissor + viewport math (renderer.setScissor / renderer.setViewport). This works on the ^0.180.0 three.js pin — no need for the ViewHelper.location.left/right/top/bottom field added in r184.
• set_axes_labels honors the labels tuple via viewHelper.setLabels(...) and forwards font/color/radius to viewHelper.setLabelStyle(...) (a public, JSDoc-documented method on r180 with defaults '24px Arial' / '#000000' / 14). Toggling the inset off then on re-applies the cached labels and style on the rebuilt ViewHelper.
• Click an axis to snap the camera: pointer events on the canvas are forwarded to viewHelper.handleClick(event) via a capture-phase listener. r180's handleClick has a hardcoded dim = 128 and assumes the inset is at the canvas's bottom-right, so we synthesize event coordinates that make its math compute the correct NDC for our actual inset rect (works for any anchor/size). When the click hits an axis, the listener calls event.stopImmediatePropagation() so OrbitControls/DragControls don't also see the click and start a drag-rotate. Subsequent click3d event emission is suppressed for that pointerdown so apps don't see a stray click on whatever the underlying scene happens to be at the inset's screen rect.

Named click intersections (Scene(intersection_planes=[...]), SceneClickEventArguments.intersections):

• New SceneIntersectionPlane(name, axis: 'x' | 'y' | 'z', offset) dataclass and ScenePoint(x, y, z) dataclass.
• Scene constructor gains intersection_planes: list[SceneIntersectionPlane] | None = None. Empty list / None → empty intersections dict on every click → zero per-click cost for opted-out users.
• JS-side: pre-computes a THREE.Plane per spec at mount and on prop change (with cloned axis-normal vectors so the per-axis singleton is never mutated), then on every click does raycaster.ray.intersectPlane(plane, point) per configured plane.
• SceneClickEventArguments.intersections: dict[str, ScenePoint | None] — every configured plane appears in the dict on every click; the value is None when the ray misses, so callers can distinguish "plane not configured" from "plane didn't intersect" without .get() ambiguity.

Raycaster threshold (Scene(raycaster_threshold=...)):

• Now a real watched prop. Runtime changes to the line / points hit-test threshold take effect without remounting the scene.

Tests (tests/test_scene.py):

• test_set_clipping_planes — apply + clear, assert each material's clippingPlanes array is populated then cleared.
• test_set_axes_inset_and_labels — toggle on, set custom labels with non-default font/color/radius, assert the cached opts are reflected on the JS side, toggle off then on, assert the cached labels/style survive the rebuild.
• test_axes_inset_handle_click_snaps_camera — dispatch a pointerdown over the +X axis sprite at its computed screen position, assert viewHelper.animating flips on, then off when the snap-animation completes.
• test_intersection_planes_in_click_event — configure 'ground' and 'wall' planes, click the canvas, assert both names appear in e.intersections.
• test_raycaster_threshold_runtime_change — change the prop after mount and assert the raycaster's Line.threshold updates.

Documentation (website/documentation/content/scene_documentation.py): three new demos covering clipping planes, the axes inset (now showing the click-to-snap interaction and font/color/radius), and click-plane intersections.

Progress

• The PR title is a short phrase starting with a verb like "Add ...", "Fix ...", "Update ...", "Remove ...", etc.
• The implementation is complete. (Otherwise, open a draft PR.)
• This PR does not address a security issue. (Security fixes must be coordinated via the security advisory process before opening a PR.)
• Pytests have been added.
• Documentation has been added.
• No breaking changes to the public API. (SceneClickEventArguments gains an intersections field with a default-factory empty dict.)