docs: add viable open character-tool candidates

[WebCrew]

Summary

• add MakeHuman / MPFB as an open character-foundation candidate
• add Blender as the preferred mature sculpting and asset round-trip candidate
• record licensing boundaries between CC0 assets, GPL/AGPL code and Parlyn's MIT core
• deliberately exclude legacy Sculptris and proprietary AccuRIG integration until they meet Parlyn's maintenance and licensing requirements

Documentation-only Idea Pool update. No roadmap commitment, dependency, or implementation is introduced.