Diagnose the local Emily runtime installation.
The task checks the host platform, active MLX variant, required
priv/ artifacts, NIF loadability, and a tiny Emily backend smoke
test. Checks short-circuit: when a prerequisite fails, dependent
checks are reported as [skip] instead of running and producing
cascading noise.
Flags
--help Print this help text and exit.
--variant Override the active variant (`aot` or `jit`) for
this run, e.g. `--variant jit`. Does not mutate
config — used to ask "would this host satisfy
:jit?".