Optional ReadonlyisCamera projection mode at build time — mirrors LineTSLConfig.
When true, only the ortho pixel-width branch is emitted; when
false or undefined, only the perspective branch.
Optional ReadonlyjoinJoin style at degree-2 polyline joints (#790) — build-time, mirroring
LineTSLConfig.join. MUST be resolved from the same authored attribute
the visual factory gets: the pick pass builds the same screen-space quad,
so a divergence makes a mitred corner unpickable.
Per-build configuration for the line-pick factory.