`prepareDevelopmentProject()` resolved the build target from the host
platform and architecture, so a Linux host threw `unsupported target
linux-x64` while writing the runtime descriptor. Pass the target in
instead: the launcher supplies the target it prepared, and isolated tests
supply a fixed supported target.
Extract `desktopTargetPlatform()` into desktop-build-paths.mjs so the
target-to-payload mapping has one home, and use it for the qualification
runner, which previously derived both its target directory and its
descriptor from the host architecture.
Correct the README limitation to name the module that now performs the
architecture check.
mektpoy