Resolves the WIP soundness bug. Root cause: the char-classes emitted for cofactor conditions were non-canonical and unrecognized downstream (of_pred(lambda) -> opaque select(lambda,ele); raw range unions carried (and true RANGE) that is_char_const_range rejected). Fix: mk_charclass_re maps the cofactor condition (true/false/and/or/not/=/char.<=) to the canonical seq::range_predicate via its Boolean ops, then range_predicate_to_regex (seq_range_collapse.h) yields a canonical char-class regex the derivative/emptiness/primitive path handles natively. Also: derive_of_pred beta-reduces select(lambda,ele) (general of_pred soundness fix).
RESULT on gen-lb (119): 0 default disagreements, 0 nseq spurious-unsat. L15 negcount family (27) timeout->solved: e.g. l15-digit-m3-dash now sat in 0.02s (dfs-nodes 33071->55, split complement/max-split-set eliminated). No regression (L11 sat, L14 unsat). Validates the notes.md derivative-based split redesign for star-free complements.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
