`compile_error(msg)` raises a build-time diagnostic at the call site
with `msg` as the error text. The arg must be a string literal —
runtime expressions can't be reported as compile errors. Used by
builder fns to reject malformed pack shapes / arg combinations
cleanly instead of silently emitting wrong code.
Today: `unresolved 'compile_error'`. Expected (post-fix): focused
diagnostic with the literal message at the call site's span. The
next commit adds the lowering arm.
82f291e5af