Make V scripts canonical for generation and builds

[antono3]

Make generate.vsh the canonical binding generator instead of a wrapper around Bash, and remove the obsolete generate_v.sh and build_vimgui.sh entry points. Maintainer documentation and the upstream-update caller now use V directly.

Validation:

• stable V 0.5.2 syntax check
• current V master/V3 format and syntax check
• v run generate.vsh --help
• full v run generate.vsh translation, combined cleanup, formatting, and native CMake build
• regenerated tracked bindings and headers are byte-for-byte unchanged
• no remaining references to either removed root Bash script

The platform-specific setup/demo helpers and the packaged binary launcher remain separate scripts because they are not duplicate implementations of these V entry points.