fix(derived): carry the placement sequence in mpe_derived

Same defect as the core review found in mp_derivedvars: the final assemble
merged the placement table by name, but that table is built in dependency
rounds and is not name ordered, so the BY-merge failed whenever the rounds
were not alphabetical, and seq was a BY-group counter rather than the
dependency order.

Each round now stamps seq=_n_+nplaced, the merge runs on the name-sorted
tables, and the result is sorted by seq.  Also guards the physical-column
in-list for a zero-column table, and checks libds exists before running.

sas/sasjs/macros/mpe_derived.sas

@@ -77,7 +77,7 @@
   outview=0
 );
 
-%local physlist derived_err resv_list i w left nready
+%local physlist derived_err resv_list i w left nready nplaced
   cyc_start cyc_cur cyc_next cyc_path cyc_seen cyc_i;
 
 %mp_abort(iftrue= (%length(&libds)=0 or %length(&dclib)=0)
@@ -88,6 +88,10 @@
   ,mac=&sysmacroname
   ,msg=%str(Please provide an outref for the generated statements)
 )
+%mp_abort(iftrue= (%mf_existds(libds=&libds) ne 1)
+  ,mac=&sysmacroname
+  ,msg=%str(Target table not found: &libds)
+)
 
 filename &outref temp;
 
@@ -146,8 +150,10 @@ data work.derived_bad;
     then reason='VAR_NM must be a valid SAS name';
   else if verify(substr(name,1,1),'0123456789')=0
     then reason='VAR_NM must not start with a digit';
+  %if %length(&physlist)>0 %then %do;
   else if name in (%unquote(&physlist))
     then reason='VAR_NM collides with a column in the target table';
+  %end;
   else if name in (%unquote(&resv_list))
     then reason='VAR_NM is reserved';
   else if type not in ('C','N')
@@ -322,13 +328,15 @@ data work.derived_todo(keep=name);
   set work.derived_byname;
 run;
 
-data work.derived_placed(keep=name);
+data work.derived_placed(keep=name seq);
   length name $32;
   name='';
+  seq=0;
   if 0;
 run;
 
 %let left=%mf_nobs(work.derived_todo);
+%let nplaced=0;
 
 %do %while (&left>0);
   proc sql;
@@ -376,8 +384,15 @@ run;
     )
   %end;
 
+  /* seq carries the dependency order - the placement rounds are not name
+    ordered, so the final merge must not rely on by name alone */
+  data work.derived_ready;
+    set work.derived_ready;
+    seq=_n_+&nplaced;
+  run;
   proc append base=work.derived_placed data=work.derived_ready force;
   run;
+  %let nplaced=%eval(&nplaced+&nready);
 
   proc sql;
   delete from work.derived_todo
@@ -387,11 +402,18 @@ run;
   %let left=%mf_nobs(work.derived_todo);
 %end;
 
+proc sort data=work.derived_placed;
+  by name;
+run;
+
 data work.derived_srt;
   merge work.derived_placed(in=in1) work.derived_byname(in=in2);
   by name;
   if in1 and in2;
-  seq=_n_;
+run;
+
+proc sort data=work.derived_srt;
+  by seq;
 run;
 
 /**