[workspace.build] defines are lost for sibling path dependencies #690

Summary

[workspace.build] defines reaches a workspace member built as the selected root, but does not reach the same member when it is a sibling path dependency. The documented inheritance rule says vectors including defines are inherited by every member. In contrast, [workspace.build] cxxflags = ["-D..."] works in both positions.

Environment

• mcpp 2026.9.24.1 (repository commit b4824697)
• Windows x86_64, LLVM 20.1.7, MSVC system sysroot 14.51.36231

Minimal reproduction

Root mcpp.toml:

[workspace]
members = ["lib", "app"]

[workspace.package]
version = "0.1.0"
standard = "c++23"

[toolchain]
windows = "llvm@20.1.7"

[workspace.build]
defines = ["WORKSPACE_DEFINE=1"]

lib/mcpp.toml:

[package]
namespace = "probe"
name = "lib"

[targets.probe_lib]
kind = "lib"

[build]
sources = ["lib.cpp"]

lib/lib.cpp:

#ifndef WORKSPACE_DEFINE
#error WORKSPACE_DEFINE missing in library dependency
#endif
int probe_lib() { return WORKSPACE_DEFINE; }

app/mcpp.toml:

[package]
namespace = "probe"
name = "app"

[dependencies]
"probe.lib" = { path = "../lib" }

[targets.probe_app]
kind = "bin"
main = "main.cpp"

app/main.cpp:

int probe_lib();
int main() { return probe_lib() != 1; }

From the workspace root:

1. mcpp build -p lib succeeds.
2. mcpp build -p app fails compiling lib.cpp with WORKSPACE_DEFINE missing in library dependency. The failing compile command has no -DWORKSPACE_DEFINE=1.
3. Replace defines = ["WORKSPACE_DEFINE=1"] with cxxflags = ["-DWORKSPACE_DEFINE=1"] in [workspace.build]. mcpp build -p app succeeds.

Expected

The lib member receives workspace defines in both builds, as documented in docs/07-workspace.md under the [workspace.build] merge rule.

Suspected cause

In src/build/prepare.cppm, fold_build_defines_into_flags(dep_manifest->buildConfig) runs for a path dependency before makePackageRoot calls inherit_workspace_build. The latter prepends the workspace defines after that fold, while makePackageRoot copies cflags and cxxflags to privateBuild without folding defines again. Thus inherited cxxflags reach the compiler, but inherited defines remain unused. The selected root takes a different path and works.