Fix the rendering faults on the Crust and Plate Motion screens

[veillette]

Plate Motion was drawing four things wrong, all visible on first load:

• The sea was never painted. paintMantle filled the section from sea
  level down, over the seawater rectangle drawn just before it, so
  showSeawaterProperty was a no-op and every ocean floor had sky above
  it instead of water. The mantle is now clipped to below the ground —
  the merge of both plates' crustTop polylines — which also lets a
  collision belt standing above the waterline keep mantle under it.

• The subducting plate drew as a bowtie. subductionGeometry
  reversed the down-going plate's crustTop but not its two base lines.
  fillBand closes a band by walking the top forward and the base back,
  so the mismatched direction folded the plate into a self-crossing X
  spanning half the section. All three polylines now run the same way,
  and PlateGeometry.test.ts asserts that invariant for every pairing,
  motion and time — it fails on the old code.

• Density mode, the default, showed no plates and no slab. The
  lithospheric mantle and the descending slab were both painted at the
  asthenosphere's MANTLE_DENSITY_KG_M3, so each plate appeared to be
  crust floating on nothing. They are the same rock at different
  temperatures, and the density difference is the whole reason a cooled
  plate can sink, so they now get their own values.

• A spent slab smeared along the floor. CrossSectionScale.y clamps,
  so a slab past the bottom of the view was drawn as a horizontal streak
  along the bottom edge with its arrow-heads strung out sideways. The
  centreline is trimmed at the first point below the section.

Also on that screen, the crust chooser overlapped the cross-section: the
section's top was inset by a hardcoded 62 px against a panel that is
taller than that. It is measured now, since the height depends on the
font and on the length of the localized crust names.

On Crust, a block whose surface is below sea level had its label placed
in the few pixels of water above it, landing on the sea-level line and
its caption — which is exactly where the user's own block sits at the
default thickness. Submarine blocks now carry the label just inside the
rock; blocks standing above the water keep theirs in the sky. columnAt
also treats the outermost edge of the last block as inside it, so the
right-hand edge of the viewport no longer reads as open mantle to the
painter and the probe.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01JZ1JDB8VPtJPHoiu7C33kQ

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