feat(Boole): nat/pos as first-class types with UF+axiom SMT encoding - #10
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Adds nat as a built-in Boole type. No imports or declarations required — every #strata program Boole; block has it in scope automatically.
The binary-nat algebraic datatype (pos with constructors xH/xO/xI, nat with N0/Npos) is emitted as SMT declare-datatypes with a recursive pos.toInt definition and bridge axioms (nat_nonneg, nat_fromInt_toInt, nat_toInt_fromInt). The library is injected automatically when coreProgUsesNatOrPos detects nat or pos anywhere in the program — procedure headers, spec clauses, ite conditions, loop invariants, quantifier bodies, or algebraic datatype constructor fields.
Nat operations (nat_toInt, nat_fromInt, nat_add, nat_sub, nat_mul, nat_div, nat_mod, nat_lt, nat_le, nat_gt, nat_ge) are available as surface-syntax operators in all Boole programs.
Counterexample support: when cvc5 returns a candidate model for a failing nat property, the natCandidatePhase validates and promotes it to ❌ fail. Binary pos/nat constructor terms in the model are decoded back to integers for display.
Depends on strata-org/Strata#1439 (bumped in this PR).
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