fix: prepare macOS library paths for embedded R

[eitsupi]

• Prepare DYLD_LIBRARY_PATH before embedding R on macOS so relocated user-mode installations can load their shared-library dependencies without going through the R launcher.
• Put the selected R library directory first, remove duplicate entries for that directory, and preserve other search paths so a previous R installation does not take priority.
• Carry the original DYLD environment through loader re-exec and restore it on :switch; ordinary :restart preserves current environment settings and prepares the newly selected R directory.
• Compare native R and arf without external loader-variable injection on macOS, covering normal startup, --no-environ, and --vanilla, including Renviron read counts and project package/DLL locations.
• Pin rig 0.11.0 with mise and a generated lockfile, use R 4.6.1, and separate the Pixi and rig Rust cache keys so the integration workflows can reuse artifacts from their own build conditions.