Early interp (density)

[ShawnL00]

WIP:

This PR adds need-driven interpolation for layer-potential densities.

Plain densities continue to be interpolated as whole units from stage1 to quad_stage2. For densities coupled with geometry factors, this PR adds explicit markers:

• sym.distribute_interpolation(expr) allows interpolation to be pushed into selected density expressions.
• sym.geo_density(geometry, density) is a helper for common geometry * unknown densities.
• sym.interpolation_unit(expr) marks quantities that must remain atomic under interpolation.

For example, this lets expressions such as normal * sigma be written so that the normal is formed using the stage2 geometry parameterization and interpolated to quad_stage2, while the unknown sigma is interpolated from stage1 to quad_stage2. At the same time, QWeight/L2-scaled quantities such as u / sqrt_jac_q_weight remain atomic.

tangential_to_xyz(...) transforms variables from a local tangential basis to the standard Cartesian basis, so the resulting Cartesian vector should be interpolated as an atomic quantity.