[codex] support package runtime shared dependencies

[Sunrisepeak]

Summary

Adds the mcpp build/runtime support needed for upstream-compatible compat packages that provide real shared-library artifacts, such as the X11 stack used by GLFW.

• Build dependency package shared targets as package-owned artifacts instead of flattening them into consumers.
• Link direct shared dependencies with per-link flags, implicit ninja inputs, and runtime rpath on ELF/macOS.
• Preserve static package flattening while propagating the static package's direct shared dependencies into the final link.
• Support xpkg mcpp.<platform> overlays so package descriptors can add platform-specific sources/deps/flags.
• Treat Objective-C .m files as C-like implementation sources for macOS package overlays.
• Fix local path index installs to use index:full.package.name@version, avoid unrelated builtin-index refreshes, expose the official xim index to project-scoped installs, and clear stale local xlings index caches.

Why

The compat package work needs existing C/C++ code to keep using upstream headers and runtime libraries directly, e.g. #include <X11/Xlib.h> and binaries linking libX11.so. That requires dependency packages to produce and propagate shared artifacts instead of only contributing headers or object files.

Validation

• target/x86_64-linux-gnu/76723324835d8177/bin/mcpp build
• target/x86_64-linux-gnu/76723324835d8177/bin/mcpp test -- tests/unit/test_config.cpp
• MCPP=$PWD/target/x86_64-linux-gnu/76723324835d8177/bin/mcpp bash tests/e2e/52_local_path_namespaced_index.sh
• MCPP=$PWD/target/x86_64-linux-gnu/76723324835d8177/bin/mcpp bash tests/e2e/55_dependency_shared_artifact.sh
• MCPP=$PWD/target/x86_64-linux-gnu/76723324835d8177/bin/mcpp bash tests/e2e/56_transitive_shared_artifact.sh
• MCPP=$PWD/target/x86_64-linux-gnu/76723324835d8177/bin/mcpp bash tests/e2e/57_static_dep_shared_artifact.sh
• git diff --check