Fix mjpython libpython resolution for uv and @rpath interpreters

[Mika412]

On macOS, running anything under mjpython on a uv managed Python fails with dyld: Library not loaded: @rpath/libpython3.x.dylib. The same failure occurs with any interpreter that links libpython through @rpath.

At startup, mjpython.py parses otool -l and adds the interpreter's library directories to DYLD_FALLBACK_LIBRARY_PATH so libpython resolves. Two bugs broke that for @rpath interpreters:

• It ran dirname() on every entry, but LC_RPATH values are already directories, so the real lib/ got reduced to its parent and libpython was never found.
• It didn't follow the interpreter symlink first, so for a venv (bin/python points at the base interpreter) the paths were anchored in the wrong place.

Also swaps the main-thread startup busy-wait (while (!py_initialized.load()) {}) for a dispatch semaphore, and makes the three interpreter-bootstrap calls fail loudly instead of silently continuing into a broken state.

All changes are limited to the macOS-only mjpython launcher (python/mujoco/mjpython/). Nothing else is affected.

This fixes the failure reported in #1923 (closed without a fix) and astral-sh/uv#8953.