Generate the npm publish tree that ships a prebuilt Rust binary.
The tree follows the "platform packages" pattern: a meta package whose
optionalDependencies point at one @scope/name-<os>-<cpu> package per
platform, each carrying the binary and filtered by npm os/cpu. Package
identity is read from the target crate via cargo metadata; targets, payload
and foreign manifests are declared under [package.metadata.npmgen] (or
[workspace.metadata.npmgen]).
Entry point: [Generator::builder].