Add a rotatable 3-D globe and a tectonic-plates layer toggle

[veillette]

The global map can now be drawn as an orthographic globe the user turns by
dragging (or with the arrow keys), and the per-plate colour wash has its own
checkbox alongside the other layers.

Both global views draw the same datasets in the same order and colours:
EarthCanvasNode owns everything they share, and each subclass supplies only
what depends on the shape of the world — the clip, the base map, and how a
polyline of lon/lat becomes a canvas path. Anything that draws geography,
including the plate labels and motion arrows, goes through the new
EarthProjection interface, whose project() reports whether a point is visible
as well as where it goes.

Three hazards are peculiar to the globe and are handled in GlobeCanvasNode:

• The limb. Polylines are cut where they cross it; filled outlines detour to
  a ring outside the disc and skirt round to where they reappear, which the
  disc clip trims into a clean edge.
• Long segments. The data was shaped for a flat map, where a straight line
  between vertices is right — the Pacific plate's eastern edge climbs 66° of
  latitude in one step, which on a globe is a chord straight through the
  Earth. Segments are now sampled along their great circle every 5°.
• Dataset seams. Plates that straddle ±180° are stored slit open along it,
  and polar plates closed off along the pole. Invisible at the edge of a flat
  map; bright lines up the Pacific on a globe, so they are filled but not
  stroked.

The relief raster is equirectangular, so the globe un-projects each pixel of
the disc and samples it into a texture, rebuilt only when the camera moves.

The globe's camera lives in the view and is reset by Reset All; the choice of
globe or flat map is model state, so the screen summary describes it.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_013LZwLXenFAnHm6QiVYK8Ld

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