plan: add SX primitive baseline section to CL/APL/Ruby/Tcl plans

plans/apl-on-sx.md

@@ -104,6 +104,16 @@ Core mapping:
 - [ ] Drive corpus to 100+ green
 - [ ] Idiom corpus — `lib/apl/tests/idioms.sx` covering classic Roger Hui / Phil Last idioms
 
+## SX primitive baseline
+
+Use vectors for arrays; numeric tower + rationals for numbers; ADTs for tagged data;
+coroutines for fibers; string-buffer for mutable string building; bitwise ops for bit
+manipulation; multiple values for multi-return; promises for lazy evaluation; hash tables
+for mutable associative storage; sets for O(1) membership; sequence protocol for
+polymorphic iteration; gensym for unique symbols; char type for characters; string ports
++ read/write for reader protocols; regexp for pattern matching; bytevectors for binary
+data; format for string templating.
+
 ## Progress log
 
 _Newest first._