arvo: add %uv hash case to scry paths

Clay desparately wants something like this. It helps answer questions
around hash-based content fetching.

pkg/arvo/sys/arvo.hoon

@@ -39,10 +39,12 @@
   $%  ::  %da:  date
       ::  %tas: label
       ::  %ud:  sequence
+      ::  %uv:  hash
       ::
       [%da p=@da]
       [%tas p=@tas]
       [%ud p=@ud]
+      [%uv p=@uv]
   ==
 +$  cage  (cask vase)
 ++  cask  |$  [a]  (pair mark a)
@@ -313,6 +315,7 @@
   ^-  (unit case)
   ?^  num=(slaw %ud knot)  `[%ud u.num]
   ?^  wen=(slaw %da knot)  `[%da u.wen]
+  ?^  hax=(slaw %uv knot)  `[%uv u.hax]
   ?~  lab=(slaw %tas knot)  ~
   `[%tas u.lab]
 ::

pkg/arvo/sys/lull.hoon

@@ -1754,6 +1754,7 @@
     $%  [%da p=@da]                                     ::  date
         [%tas p=@tas]                                   ::  label
         [%ud p=@ud]                                     ::  number
+        [%uv p=@uv]                                     ::  hash
     ==                                                  ::
   +$  cash                                              ::  case or tako
     $%  [%tako p=tako]                                  ::

pkg/arvo/sys/vane/clay.hoon

@@ -1451,6 +1451,7 @@
     ?-    -.lok
         %tas  (~(get by lab.dom) p.lok)
         %ud   ?:((gth p.lok let.dom) ~ [~ p.lok])
+        %uv   ?~(yak=(~(get by hut.ran) p.lok) ~ $(lok [%da t.u.yak]))
         %da
       ?:  (gth p.lok lim)  ~
       |-  ^-  (unit aeon)