guard_set: wrap rp_cache into guard_set_cache class (#10362)

Addresses code review feedback on PR #10361: the raw `obj_map` typedef +
static `dealloc_cache` helper is replaced by a proper `guard_set_cache`
class that owns its memory and adds a recycling optimization.

## Changes

- **New `guard_set_cache` class** (`guard_set.h` / `.cpp`):
- `m_cache` — `obj_map<expr, seq::range_predicate*>` (null value =
unsupported guard, avoids retranslation)
- `m_fresh` — pre-allocated `range_predicate*` recycled across failed
translations; when `guard_to_range_predicate` rejects a guard, the heap
object is retained in `m_fresh` instead of deallocated, so the next miss
reuses it without an alloc/dealloc round-trip
- Public API: `reset()`, `find(expr*, seq::range_predicate*&) → bool`,
`fresh(unsigned max_char) → seq::range_predicate*`, `insert(expr*,
seq::range_predicate*)`

- **`guard_set`**: removed `rp_cache` typedef and `dealloc_cache`
static; constructor parameter updated to `guard_set_cache*`; `conjoin`
updated to use `fresh()` + `insert()` for the recycling path

- **`seq_monadic`**: field type `guard_set::rp_cache` →
`guard_set_cache`; reset call updated to `m_rp_cache.reset()`

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Nikolaj Bjorner <nikolaj@cs.stanford.edu>

src/ast/rewriter/seq_monadic.cpp

@@ -80,7 +80,7 @@ void seq_monadic::reset_cofactor_cache() {
     for (auto const& [k, v] : m_cofactor_cache)
         dealloc(v);
     m_cofactor_cache.reset();
-    guard_set::dealloc_cache(m_rp_cache);   // the guards live in the cofactor vectors
+    m_rp_cache.reset();                      // the guards live in the cofactor vectors
 }
 
 bool seq_monadic::live_states(expr* R, expr_ref_vector& out) {