Optionalexplicit: LinePrimitive | null
a caller-supplied primitive that bypasses the session
override. Test harnesses (the GLSL/TSL parity page) never run
bootstrap.ts, so fixtures pass the primitive explicitly instead of
relying on module state.
Resolve the primitive to build, applying
?linePrimitive=> DEFAULT_LINE_PRIMITIVE. A forced (non-auto) policy replaces the default; the URL parameter stays the strongest override (the explicit A/B escape hatch).