rlmesh.params.ParamSpec

The validated ceiling over the free signature-derived floor. extra governs the single boundary door for undeclared keys:

• "forbid" (default): an undeclared key raises before construction, so a typo (robtos=) fails pre-GPU instead of vanishing.
• "passthrough": undeclared keys forward verbatim through the author’s own **kwargs into a third-party constructor (the escape hatch for a wrapper author). Bounded by that **kwargs – never by any downstream target – so it cannot collide with a body-computed argument.