fix(derived): guard the bitemporal cut-off, coalesce the selectbox seed

mpe_derived used &dc_dttmtfmt in its where= but the guard only checked libds and
dclib, so an unset value turned the filter into 'lt tx_to' and the control-table
step failed with a syntax error a long way from the cause.  It is now part of
the guard, named in the message.

The 7.19 migration took max(selectbox_rk) into a macro variable, which is
missing on an install whose MPE_SELECTBOX is still empty - %eval(&maxrk+1) then
fails.  coalesce(max(selectbox_rk),0) makes the first row start at 1.

sas/sasjs/db/migrations/20261019_v7.19_release.sas

@@ -61,7 +61,9 @@ quit;
   * further down, mirror the MPE_DERIVED rows in mpe_makedata.sas - keep the two
   * in step, or an upgraded install and a freshly seeded one behave differently. */
 proc sql noprint;
-  select max(selectbox_rk) into: maxrk from &dclib..mpe_selectbox;
+  /* coalesce: an install with no selectbox rows yet returns a missing value,
+    and %eval(&maxrk+1) below would then fail */
+  select coalesce(max(selectbox_rk),0) into: maxrk from &dclib..mpe_selectbox;
 quit;
 
 %let rk1=%eval(&maxrk+1);

sas/sasjs/macros/mpe_derived.sas

@@ -85,9 +85,12 @@
   them whatever MPE_TABLES calls them */
 %let reserved=%str(_____DELETE__THIS__RECORD_____);
 
-%mp_abort(iftrue= (%length(&libds)=0 or %length(&dclib)=0)
+/* dc_dttmtfmt is the bitemporal cut-off used in the where= below - callers get
+  it from %mpeinit().  Unset, the filter degenerates to "lt tx_to" and the
+  control-table step fails with a syntax error a long way from the cause. */
+%mp_abort(iftrue= (%length(&libds)=0 or %length(&dclib)=0 or %length(&dc_dttmtfmt)=0)
   ,mac=&sysmacroname
-  ,msg=%str(Missing: libds dclib)
+  ,msg=%str(Missing: libds dclib dc_dttmtfmt)
 )
 
 /* A format catalog is addressed as LIBREF.CATALOGNAME-FC (the form
@@ -97,13 +100,16 @@
   variable list and an empty statement file instead, and leave outview alone:
   a caller only reads the view when the list is not empty. */
 %if %length(&libds)>3 and "%upcase(%substr(&libds,%length(&libds)-2,3))"="-FC" %then %do;
+  /* the file has to exist even though it stays empty - callers %include it
+    unconditionally, and a fileref with no file behind it fails there */
   %if &outref ne 0 %then %do;
     filename &outref temp lrecl=32767;
     data _null_;
       file &outref;
     run;
   %end;
-  /* same idiom as the engine, so the dataset has the column and no rows */
+  /* the same shape the engine builds before it returns: one NAME column and no
+    rows, so a caller that reads NAME (all of them do) sees nothing */
   data &outvars(keep=name);
     length name $32;
     name='';