geo/SDFSeam

Keeps NEITHER input. What comes out is the rod of points within `radius` of both surfaces at once, which follows the curve where the two cross. It is hg_sdf's fOpPipe, also known as a pipe or a weld bead, and it is the third thing two fields can do to each other: geo/SDFBoolean keeps both inputs, geo/SDFImprint keeps the first, this keeps neither.

Each input's own objects are hard-unioned first, so a geo/Copy or geo/Merge fan-in works on either side. Non-SDF objects are skipped, and a 2D field works exactly as a 3D one does: two curves crossing at a point leave a disc there rather than a rod.

**Uses.** A weld or fillet bead along a joint, piping or trim along an edge, and a quick way to SEE where two fields actually touch, which is otherwise hard to inspect: feed both inputs the shapes you are unsure about and the rod appears exactly where they meet.

**The profile decides the rod's cross section.** ROUND is a disc of radius `radius`, SQUARE is a square of that half side. The cost runs the OTHER way round from geo/SDFBoolean's, which is worth knowing because the instinct carries over wrongly: ROUND reports up to 41% too much distance where the two surfaces run parallel, so the preview march shortens its step for the whole object, while SQUARE is built from abs and max alone and costs nothing at all. A scene of round beads may want a higher `maxStepsCount` on its mat/MeshSDF. geo/SDFTriangulate only samples, so the meshed result is exact for either profile.

**Per-region colours come with it.** Put a geo/MaterialAttributes on each input, leave its mode on OBJECT, and the bead fades from one input's colour to the other's around its own cross section, since the rod runs from the side hugging the first surface to the side hugging the second. geo/SDFTriangulate carries those values onto the meshed points for multi-colour export.