Fix DU<, register 2VARIABLE/2CONSTANT callable — double 27→3

- DU< had same comparison order bug as D< (comparing d2-hi < d1-hi
  instead of d1-hi < d2-hi). Fixed with SWAP U<.
- 2VARIABLE and 2CONSTANT were handled as special tokens but not
  registered in the dictionary, so they couldn't be called from
  compiled code (e.g., : CD4 2VARIABLE ;). Added pending codes 9/10.

crates/core/boot.fth

@@ -118,7 +118,8 @@
 : M+  S>D D+ ;
 
 \ DU< ( ud1 ud2 -- flag )  unsigned double-cell less-than
-: DU<  ROT 2DUP = IF 2DROP U< ELSE U< NIP NIP THEN ;
+\ Compare high cells (unsigned); if equal, compare low cells unsigned.
+: DU<  ROT 2DUP = IF 2DROP U< ELSE SWAP U< NIP NIP THEN ;
 
 \ ---------------------------------------------------------------
 \ Phase 3: Mixed arithmetic (built on M* and UM/MOD host primitives)