feat(plotting): add plot_surface for displaying meshes in napari - #263

Closes #262. Closes #335.

Stacked on #448 (multi-region get_mesh and batched mesh prefetch), which is stacked on #446.

What

Adds two napari helpers for meshes, the mesh counterpart to plot_napari:

• confusius.plotting.plot_surface(mesh, ...): a thin wrapper over napari.Viewer.add_surface taking a bare (vertices, faces) tuple, with optional per-vertex values for colormapped shading.
• confusius.plotting.plot_atlas_mesh(atlas, regions, sides, ...): the atlas-aware version, which pulls the meshes, per-vertex colors, layer name, axis labels, and units from the atlas Dataset before handing off to plot_surface. Also reachable as ds.atlas.plot.mesh(...), through a new AtlasPlotAccessor hanging off the .atlas namespace.

import confusius as cf

atlas = cf.datasets.fetch_brainglobe_atlas("allen_mouse_25um")
viewer, _ = cf.plotting.plot_napari(atlas.atlas.reference)
cf.plotting.plot_atlas_mesh(atlas, ["VISp", "AUDp", "MOp"], viewer=viewer)

Both switch the viewer to 3D (ndisplay = 3): in 2D a Surface layer is drawn only as its cross-section with the current slice plane, which leaves a 3D mesh all but invisible.

Because get_mesh returns vertices already in the atlas physical space, and image layers sit at their physical origin and spacing, surfaces line up with the image layers without any scale/translate juggling.

Multiple regions per call (#335)

plot_atlas_mesh takes the same regions/sides arguments as the multi-region get_mesh from #448 and merges every requested region into a single Surface layer, giving each region its own atlas color through the layer's vertex_colors. An explicit colormap, or per-vertex values, still overrides that. The layer is named after the region (full name plus acronym) when a single region is requested, and after the requested acronyms otherwise.

Tests

• TestPlotSurface and TestPlotAtlasMesh in tests/unit/test_plotting/test_napari.py: mesh and values round-tripping into the layer, the flat-surface default, overlay on an existing viewer with units, name and per-vertex colors read from the atlas, hemisphere clipping, the multi-region merge (face re-indexing and per-region colors), and accessor delegation.

Docs

• Changelog entry under 0.7.1.dev0.
• "Region Surface Meshes" in the atlas user guide now shows plot.mesh for display.
• New row in the visualization user-guide tool table. The API reference pages pick both functions up via __all__.