Prevent reentrant PDOStatement operations

[iliaal]

PDO core runs userland PHP while holding or mutating a statement's live binding and fetch state (object parameter stringification, bound-value destructors, error/fetch/output handlers). That code can reenter the same statement's execute() or bind*() and free the hash or parameter the outer call is still using, a use-after-free in pure PDO core affecting every driver (confirmed on pdo_sqlite and pdo_odbc under debug+ASan, USE_ZEND_ALLOC=0). Minimal reproducer, UAF in really_register_bound_param:

class Reenter {
    public PDOStatement $s;
    public function __toString(): string {
        $this->s->execute([]);
        return "x";
    }
}

$pdo = new PDO('sqlite::memory:');
$stmt = $pdo->prepare('SELECT ?');
$r = new Reenter();
$r->s = $stmt;
$stmt->bindParam(1, $r);

A per-statement guard rejects reentry into any binding or fetch operation already in progress, throwing an Error; it holds across Fiber suspension and invalidates weak references before callback-capable teardown. The flag reuses a spare pdo_stmt_t reserved bit, so size and offsets are unchanged; since that edits the driver-facing header, master only.