Add SMT-LIB choice support via array OP_CHOICE and instantiate choice axioms in array solvers (#9649)

This change wires SMT-LIB Hilbert choice parsing to a concrete
array-theory operator and ensures both array backends enforce the
expected semantic axiom. Previously, `(choice ((x T)) phi)` parsed as
NYI and had no solver-side instantiation path.

- **Parser: lower `choice_k` into array `OP_CHOICE`**
- `pop_quant_frame(choice_k)` now builds `(choice p)` instead of
throwing.
- Added parser include/use of array utilities to construct the term
directly from the generated lambda predicate.

- **Array decl plugin: add `OP_CHOICE` typing + surface syntax**
  - Added declaration support for `choice` with signature:
- `(Array T Bool) -> T` (encoded as `('a -> Bool) -> 'a` in HO view).
- Added recognizer/util helpers (`is_choice`, `mk_choice`) and exposed
`"choice"` in op names.

- **SMT array theory (`theory_array_full`): instantiate choice axiom**
  - Added instantiation for each encountered `choice(p)`:
    - `forall x . p(x) => p(choice(p))`
  - Integrated into internalization/relevancy paths and statistics.

- **SAT/SMT array backend (`sat/smt/array_*`): instantiate choice
axiom**
- Added new axiom record kind for choice, internalization hook,
assertion routine, and diagnostics/stat tracking.
  - Uses the same quantified implication schema as above.

- **Regression coverage**
- Extended SMT2 parser regression with an HO `choice` example to ensure
parser/eval pipeline accepts and processes choice terms.

Example of the now-supported input:

```smt2
(set-logic HO_ALL)
(declare-sort U 0)
(declare-fun P () (-> U Bool))
(assert (exists ((x U)) (P x)))
(assert (= witness (choice ((x U)) (P x))))
```

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

src/smt/theory_array_full.cpp

@@ -271,7 +271,7 @@ namespace smt {
             return theory_array::internalize_term(n);
         }
 
-        if (!is_const(n) && !is_default(n)  && !is_map(n) && !is_as_array(n)) {
+        if (!is_const(n) && !is_default(n)  && !is_map(n) && !is_as_array(n) && !is_choice(n)) {
             if (!is_array_ext(n))
                 found_unsupported_op(n);
             return false;
@@ -329,6 +329,9 @@ namespace smt {
             ctx.push_trail(push_back_vector(m_as_array));
             instantiate_default_as_array_axiom(node);
         }
+        else if (is_choice(n)) {
+            instantiate_choice_axiom(node);
+        }
         else if (is_array_ext(n)) {
             SASSERT(n->get_num_args() == 2);
             instantiate_extensionality(ctx.get_enode(n->get_arg(0)), ctx.get_enode(n->get_arg(1)));
@@ -406,7 +409,7 @@ namespace smt {
     void theory_array_full::relevant_eh(app* n) {
         TRACE(array, tout << mk_pp(n, m) << "\n";);
         theory_array::relevant_eh(n);
-        if (!is_default(n) && !is_select(n) && !is_map(n) && !is_const(n) && !is_as_array(n)){
+        if (!is_default(n) && !is_select(n) && !is_map(n) && !is_const(n) && !is_as_array(n) && !is_choice(n)){
             return;
         }
         ctx.ensure_internalized(n);
@@ -442,6 +445,9 @@ namespace smt {
         else if (is_as_array(n)) {
             instantiate_default_as_array_axiom(node);
         }
+        else if (is_choice(n)) {
+            instantiate_choice_axiom(node);
+        }
     }
 
     bool theory_array_full::should_research(expr_ref_vector & unsat_core) {
@@ -596,6 +602,32 @@ namespace smt {
         return try_assign_eq(val.get(), def);
     }
 
+    bool theory_array_full::instantiate_choice_axiom(enode* ch) {
+        if (!ctx.add_fingerprint(this, m_choice_fingerprint, 1, &ch))
+            return false;
+        ++m_stats.m_num_choice_axiom;
+        SASSERT(is_choice(ch));
+        app* choice_term = ch->get_expr();
+        expr* pred = choice_term->get_arg(0);
+        sort* pred_sort = pred->get_sort();
+        SASSERT(is_array_sort(pred_sort));
+        SASSERT(get_array_arity(pred_sort) == 1);
+        SASSERT(m.is_bool(get_array_range(pred_sort)));
+        sort* x_sort = get_array_domain(pred_sort, 0);
+        expr_ref x(m.mk_var(0, x_sort), m);
+        expr* args1[2] = { pred, x };
+        expr_ref px(mk_select(2, args1), m);
+        expr* args2[2] = { pred, choice_term };
+        expr_ref pc(mk_select(2, args2), m);
+        expr_ref ax(m.mk_implies(px, pc), m);
+        symbol x_name("x");
+        expr_ref q(m.mk_forall(1, &x_sort, &x_name, ax), m);
+        ctx.get_rewriter()(q);
+        literal l = mk_literal(q);
+        assert_axiom(l);
+        return true;
+    }
+
     //
     // Assert axiom:
     // select(const v, i_1, ..., i_n) = v
@@ -839,5 +871,6 @@ namespace smt {
         st.update("array def as-array", m_stats.m_num_default_as_array_axiom);
         st.update("array sel as-array", m_stats.m_num_select_as_array_axiom);
         st.update("array def lambda", m_stats.m_num_default_lambda_axiom);
+        st.update("array choice ax", m_stats.m_num_choice_axiom);
     }
 }