Add OP_RE_XOR and union-find bisimulation for ground regex equivalence (#9804)

Implements the algorithm of Eq(p,q) = Empty(p XOR q)' using a union-find
driven bisimulation closure (per the CAV'26 ERE paper).

### What's added

* **New primitive OP_RE_XOR (re.xor)** wired through seq_decl_plugin:
parser signature, info propagation (nullable, min_length), and
pretty-printer.
* **seq_rewriter**: structural XOR rewrites ( XOR r = empty, XOR empty =
r, ull XOR r = comp(r), comp/comp absorption, complement push, AC
normalisation), nullability (Null(p XOR q) = Null(p) != Null(q)),
derivative (D_a(p XOR q) = D_a(p) XOR D_a(q)), reverse, antimirov
derivative, and `check_deriv_normal_form` coverage.
* **New class seq::regex_bisim** in
`src/ast/rewriter/seq_regex_bisim.{h,cpp}` to keep the bisim logic out
of the already-large `seq_rewriter.cpp`. Uses `basic_union_find` from
`util/union_find.h`, an `obj_map` for the node assignment, and a
50000-step bound (returns `l_undef` on overrun).
* **Integration** in `seq_rewriter::reduce_re_eq` (with a re-entry
guard) and in `seq_regex::propagate_eq` / `propagate_ne` for ground
regexes; on `l_undef` we fall back to the existing axiomatisation.
* **`sls_seq_plugin`**: extend `OP_RE_DIFF` switch arms to also cover
`OP_RE_XOR`.

### Validation

* Full release build with MSVC + Ninja.
* `./test-z3 /a` -- 89/89 tests passing.
* `./test-z3 /seq smt2print_parse` -- PASS.
* Smoke tests with `(a|b)*` vs `(a*b*)*` (equal) and `a*` vs `(a|b)*`
(not equal) return the expected `sat`/`unsat` quickly.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/ast/sls/sls_seq_plugin.cpp

@@ -511,6 +511,7 @@ namespace sls {
             case OP_RE_CONCAT:
             case OP_RE_UNION:
             case OP_RE_DIFF:
+            case OP_RE_XOR:
             case OP_RE_INTERSECT:
             case OP_RE_LOOP:
             case OP_RE_POWER:
@@ -1294,6 +1295,7 @@ namespace sls {
         case OP_RE_CONCAT:
         case OP_RE_UNION:
         case OP_RE_DIFF:
+        case OP_RE_XOR:
         case OP_RE_INTERSECT:
         case OP_RE_LOOP:
         case OP_RE_POWER: