Covers the pure double-cell data-stack shuffle words from
src/word_source/double_words.c. Deliberately scoped to exclude:
- 2>R/2R>/2R@: branch on vm->ecw_nesting, a field vm_state doesn't track
at all -- needs a model extension first, not attempted here.
- S>D/D+/D-/DNEGATE/DABS/DMAX/DMIN/D</D=/D0=/D0</D2*/D2/: depend on
cell_t being a fixed-width (64-bit) wrapping integer (explicit
unsigned-long carry/borrow arithmetic, bitwise complement with
wraparound). StarForth_Base.thy's "cell = int" is unbounded, not
fixed-width, so this isn't expressible as currently modeled. Fixing it
means deciding whether cell becomes a 64-bit word type everywhere
(ripples into all 23 already-verified theories) -- a foundational
decision, flagged for later, not made as a side effect of this file.
24 theory files now verify with zero errors.
