(U+6042): Add `<-denotational@usage' for BUCS+6042; add `sound@ja/on'

(U+6042): Add `<-denotational@usage' for BUCS+6042; add `sound@ja/on'
and `sound@ja/on/conventional'.
(UU+6042): Unify DJT-03264; add some relations for other characters.
(U+609B): Add `<-denotational@usage' for BUCS+609B; add
`ideographic-structure'; add `sound@ja/on'.
(CU+609B): Add `ideographic-structure'; add some relations for other
characters.
(UU+609B): Add `ideographic-structure'; unify DJT-03324.
(BUCS+6042): New abstract node; unify A2-DJT-03264.
(BUCS+609B): New abstract node; unify A2-DJT-03324.