Preserve standalone markers in workspace metadata (#20011)
## Summary
#19971 simplifies locked dependency markers under parent reachability.
`uv workspace metadata` was still exporting those contextual markers
directly, even though metadata edge markers must stand alone. If a
package was reachable only on Linux, an unconditional child edge could
lose its `sys_platform == 'linux'` marker and be reported as "always
required".
This PR reconstructs standalone metadata edge markers by propagating
reachability from workspace and script entry points.
