A workspace root's xlings declaration is not visible to its members' build programs, so each member repeats it #713

Observed

A workspace root that declares an xlings payload does not make that payload visible to its members' build programs. A member that uses the payload through mcpp::xpkg_dir (for instance, rules-qt locating the Qt SDK) has to declare the payload itself, so the same version is written once per member.

Reproduction (mcpp 2026.9.26.2, Linux): a workspace with members app and tool, each of whose build programs calls mcpp::rules::qt::compile(), and a root manifest that declares:

[target.'cfg(any(windows, linux, macos))'.xlings.workspace]   # also tried: [target.linux.xlings.workspace], [xlings.workspace]
"xim:qt-base" = "6.11.1"

mcpp build -p tool reports that the member sees no payload:

warning: tool: mcpp.rules.qt: no Qt SDK. Nothing Qt-specific is planned. …
fatal error: QtCore/QtGlobal: No such file or directory

The payload is installed. When each member declares the same entry itself, the build passes.

docs/23-the-project-environment.md says that "in a workspace build the workspace root owns the selection". docs/07-workspace.md says that a root [target.<triple>] block "is already inherited by every member". Both statements describe the opposite of what is observed.

GalTranslPP writes "xim:qt-base" and "xim:7zip" in four member manifests for this reason.

Request

One of the following:

• a root declaration is visible, through xpkg_dir, to the build programs of the members;
• xlings entries support the explicit opt-in that [workspace.dependencies] already has for mcpp dependencies, so that a member writes "xim:qt-base".workspace = true and names the version in one place.