Port is_classical attribute to seq_util::rex::info (#9796)

`is_classical` (tracks whether a regex uses only classical operators —
no complement, intersection, diff, or empty-language/fail) was only
available on `euf::snode`. Moving it into `seq_util::rex::info` makes it
accessible to all regex-handling code without going through the snode
layer.

### Changes

**`seq_decl_plugin.h`**
- Added `bool classical { true }` to `seq_util::rex::info`
- The general `info` constructor requires `bool is_classical` explicitly
(no default)

**`seq_decl_plugin.cpp`**
- `mk_info_rec`: `OP_RE_EMPTY_SET` (fail) sets `classical=false`
- `mk_info_rec`: `OP_RE_RANGE`, `OP_RE_FULL_CHAR_SET`, `OP_RE_OF_PRED`
set `classical=false`
- `complement()`, `conj()` (intersection), `diff()`: always produce
`classical=false`
- `star()`, `plus()`, `opt()`, `concat()`, `disj()`, `orelse()`,
`loop()`: propagate `classical` via logical AND over operands
- `operator=` and `display()` updated to include `classical`

### Semantics

| Operation | `classical` |
|-----------|-------------|
| `re.empty` (fail) | `false` |
| `re.range`, `re.allchar`, `re.of.pred` | `false` |
| `re.comp` (complement) | `false` |
| `re.inter` (intersection) | `false` |
| `re.diff` | `false` |
| `re.all` (full sequence set) | `true` |
| `str.to.re` (string literal) | `true` |
| `re.*`, `re.+`, `re.opt`, `re.++`, `re.union`, `re.loop` | inherited
from operands |

---------

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

src/ast/seq_decl_plugin.h

@@ -443,6 +443,8 @@ public:
             lbool nullable { l_undef };
             /* Lower bound  on the length of all accepted words. */
             unsigned min_length { 0 };
+            /* Classical regular expression: does not use complement, intersection, diff, or the empty language (fail). */
+            bool classical { true };
 
             /*
               Default constructor of invalid info.
@@ -459,11 +461,13 @@ public:
             */
             info(bool is_interpreted,
                 lbool is_nullable,
-                unsigned min_l) :
+                unsigned min_l,
+                bool is_classical) :
                 known(l_true), 
                 interpreted(is_interpreted),
                 nullable(is_nullable),
-                min_length(min_l) {}
+                min_length(min_l),
+                classical(is_classical) {}
 
             /*
               Appends a string representation of the info into the stream.