Add a seafloor age (isochron) layer

[veillette]

Adds a checkbox that draws the isochrons of the ocean floor — the lines
along which the crust is all one age, at 10, 20, 40 … 180 Ma — coloured
red at the young end and blue at the old one.

The point of the layer is the seafloor spreading argument: the youngest
crust runs a degree or two either side of every spreading ridge, the same
age turns up at about the same distance on both flanks, and it all stops
at about 180 Ma because everything older has been subducted.

Data comes from the EarthByte present-day age grid (Seton et al., 2020),
a 6-arc-minute netCDF-3 grid built from marine magnetic anomalies dated
against the geomagnetic polarity timescale. The new seafloor-age build
step contours it with marching squares on a 0.3° mesh and simplifies the
result to 229 polylines, and two new script modules carry the work: a
minimal classic-netCDF reader alongside the existing GeoTIFF one, and a
marching-squares contourer that skips cells with missing corners so a
contour ends at the coast instead of running onto land.

Each isochron vertex carries the plate that carries it, so the layer
rides the reconstruction: run the clock back and matched pairs walk
towards the ridge that made them, while crust younger than the
reconstruction has reached is not drawn at all, because it did not exist.

The layer is added to EarthCanvasNode, so it appears on the flat map and
the globe alike, and starts switched off. tests/geophysicalData.test.ts
asserts the physics: across the Atlantic at 24° N every isochron crosses
on both flanks, each older one further out than the last, and the two
flanks stay within a few degrees of symmetric about the axis.

Layer-panel spacings are tightened slightly so the extra checkbox does not
push the geological-time controls off the bottom of the screen.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01MjMJZcbUDbkZX5VCrTjzNq

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