Interactive rendering defaults for PyVista and Plotly (fixes #337)

[SLT2k]

Summary

Addresses every point in #337 (default rendering configuration tuning for interactive
plots), plus several rendering defects found while testing on a campus case (19k faces,
9,000 vegetation points) and a denser 800 m neighbourhood case (72k faces).

PyVista axes

• Nice-number 1/2/5 tick locator replaces the fixed 9-point linspace: round tick
  labels with adaptive decimals, at most 8 per axis.
• The z axis gets intermediate ticks; its base tick is dropped (its label collided
  with the y labels at the shared corner). The exclusion is span-relative because STL
  float noise puts nominally flat ground fractionally below zero.
• z ticks, labels and title are drawn along a screen-level direction: pure -x
  projects diagonally under the default camera, and diagonal tick stubs read as kinks
  on the (exactly vertical) z-axis line.
• All axis text is centre-justified on its anchor (the default left/bottom
  justification visibly offset labels from their ticks, worst on the x axis); x/y
  labels hang below their tick tips; the duplicated x/y "0" labels merge into a
  single label below the shared origin corner.
• Labels and titles sit further from the axes, and fonts scale with the trame still
  ratio so supersampled notebook frames keep their apparent text size.

Rendering quality (PyVista)

• 16-sample MSAA anti-aliasing (FXAA fallback for GL stacks without multisample
  support) and scene-scaled SSAO contact shading.
• Notebook stills stream at 1.5x with JPEG quality 95: ratio-1 streaming is
  undersampled on any display with OS scaling above 100%.

Ground/building classification (both backends)

• Faces now count as ground within a 1 mm height tolerance instead of a strict
  z > 0 test. STL float noise scattered most ground triangles into the buildings
  group, painting the ground in the building colour and leaving mismatched halos
  around building bases.

Vegetation

• plot_veg draws each vegetation point as an opaque voxel filling its grid cell
  (stretched z spacings included), replacing size-2, opacity-0.2 dots that were
  invisible against the geometry; color and opacity are keyword arguments.
• New plot_veg_outline (UDVis method + sim.plot_veg_outline wrapper): the same
  vegetation voxels over the outline-style base - the most readable vegetation view
  on fine meshes. Reuses the outline scene and vegetation loading via shared helpers
  rather than duplicating them.

Wireframes and lines

• Full-wireframe overlays render thin and semi-transparent in PyVista, so fine
  meshes no longer merge into a solid black mass. LineSet gains an opacity
  field plus optional Plotly-specific width/opacity overrides: Plotly's WebGL
  lines rasterize too differently for shared values, so Plotly keeps its original
  bold opaque look.
• The PyVista mesh path now honours MeshPrimitive.opacity (previously
  Plotly-only).

Plotly

• Directional Lambert shading is baked into per-face colours: plotly.js does not
  apply its lighting model to Mesh3d facecolor arrays, so uniformly coloured
  meshes rendered as unshaded silhouettes.
• Explicit axis lines (Plotly 3-D axes draw none by default, leaving the z axis
  invisible) with outside ticks long enough to keep labels clear of the plotted
  field.

API hardening

• UDBase.plot_veg/plot_trees/plot_veg_outline forward keyword arguments to
  the visualization facade; plot_trees(backend=...) previously raised
  TypeError.

Testing

• tools/python/tests/test_udbase_vis.py: 44 tests including 15 new ones (tick
  locator, z-tick policy, voxel mesh geometry incl. watertightness, Plotly axis
  contract, alias forwarding, plot_veg_outline render path); PyVista render paths
  exercised off-screen on both backends.
• Full unittest discover over tools/python/tests matches the clean-master
  baseline locally (the only reds are pre-existing Windows-environment View3D
  config tests, identical without this change).
• Verified interactively on cases with 19k and 72k facets in the trame notebook
  widget, the native desktop window, and the browser (Plotly), including a case
  without vegetation for the graceful-degradation paths.

Fixes #337

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