Fixes#2487. `npm i -g bun` puts POSIX/cmd/ps1 shims on %PATH% but never
bun.exe — and the gbrain.exe shim that `bun link` generates resolves bun.exe
specifically, so link succeeds and every gbrain call dies with bun's
misleading "bun is not installed in %PATH%" (which suggests installing a
second parallel bun). The D19 validation failure paths now detect the
condition on Windows and print the actual remediation: bun's own
process.execPath IS the hidden bun.exe — add its directory to PATH.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>