Honor mask extrude thickness beyond paint depth

[leonardoaraujosantos]

Summary

• Project field extrude samples back to the source surface before reading the painted mask, so paint depth no longer caps wall height.
• Grow voxel walls from masked surface cells along estimated normals for the requested number of layers, while preserving source colours.
• Document the surface-anchored mask behavior in the API and OpenSpec.

Regression coverage

• Measure 0.05, 0.1, and 0.6-unit field walls along three normals, including near the mask edge, and bound height variation across the patch.
• Measure voxel wall height at the same thicknesses and compare a 0.6-unit voxel wall to the field extract within one voxel.
• Run all C++ and C ABI mask extrude tests: 17 cases, 33,710 assertions passed.
• Validate the OpenSpec change and run git diff --check.

Fixes #660. ClaySpaceDesktop#178 can remove its documented cap and pin the next ClayCore release after this lands.