slang --ast-json: find each array's element read/write sites by symbol addr

To list who reads and writes an unpacked array in slang's JSON AST (run with --disable-instance-caching so every instance body is expanded), first find the Variable/Net declaration by walking Instance/GenerateBlock names and take its numeric "addr". Every use is a NamedValue whose "symbol" string is "<addr> <name>", so match it on the addr prefix, not the name. An element access is a NamedValue that is the "value" child of an ElementSelect, whose "selector" holds the index; it is a write when it sits under an Assignment's "left" (and under ElementSelect.selector it is always a read). Group sites by the enclosing ProceduralBlock (procedureKind, with its SignalEvent edge/expr for the clock), ContinuousAssign or Subroutine to get ports; a bare NamedValue under a Call (subroutine "$readmemh") is a whole-array init. ROM-style arrays show up as one constant-index ContinuousAssign per element.