Fix JSON parser decoding for escaped unicode

JSON supports Unicode as both UTF8 sequences and escaped UTF16. Unicode
points U+10000 to U+10FFFF are encoded as two consecutive escaped UTF16
units known as a surrogate pair. The JSON decoder was previously
treating surrogate pairs as two individual escaped UTF16 units. In
addition, the JSON standard allows hexadecimal letters to be either
uppercase or lowercase, which the parser was not respecting.

See:
- https://www.crockford.com/mckeeman.html
- https://www.json.org/json-en.html
- https://en.wikipedia.org/wiki/UTF-16#Code_points_from_U+010000_to_U+10FFFF

Git issue: #1776

pkg/arvo/sys/zuse.hoon

@@ -4400,10 +4400,12 @@
           =*  lip
             ^-  (list (pair @t @))
             [b+8 t+9 n+10 f+12 r+13 ~]
-          =*  wow  `(map @t @)`(malt lip)
+          =*  wow
+            ^~
+            ^-  (map @t @)
+            (malt lip)
           (sear ~(get by wow) low)
-        =*  tuf  ;~(pfix (just 'u') (cook tuft qix:ab))
-        ;~(pose doq fas bas loo tuf)
+        ;~(pose doq fas bas loo unic)
       ==
     ::                                                  ::  ++expo:de-json:html
     ++  expo                                            ::  exponent
@@ -4455,6 +4457,49 @@
     ::                                                  ::  ++spac:de-json:html
     ++  spac                                            ::  whitespace
       (star (mask [`@`9 `@`10 `@`13 ' ' ~]))
+    ::                                                  ::  ++unic:de-json:html
+    ++  unic                                            ::  escaped UTF16
+      =*  lob  0x0
+      =*  hsb  0xd800
+      =*  lsb  0xdc00
+      =*  hib  0xe000
+      =*  hil  0x1.0000
+      |^
+        %+  cook
+          |=  a=@
+          ^-  @t
+          (tuft a)
+        ;~  pfix  (just 'u')
+          ;~(pose solo pair)
+        ==
+      ++  quad                                          ::  parse num from 4 hex
+        (bass 16 (stun [4 4] hit))
+      ++  meat                                          ::  gen gate for sear:
+        |=  [bot=@ux top=@ux flp=?]                     ::  accept num in range,
+        |=  sur=@ux                                     ::  optionally reduce
+        ^-  (unit @)
+        ?.  &((gte sur bot) (lth sur top))
+          ~
+        %-  some
+        ?.  flp  sur
+        (sub sur bot)
+      ++  solo                                          ::  single valid UTF16
+        ;~  pose
+          (sear (meat lob hsb |) quad)
+          (sear (meat hib hil |) quad)
+        ==
+      ++  pair                                          ::  UTF16 surrogate pair
+        %+  cook
+          |=  [hig=@ low=@]
+            ^-  @t
+            :(add hil low (lsh [1 5] hig))
+        ;~  plug
+          (sear (meat hsb lsb &) quad)
+          ;~  pfix  (jest '\\u')
+            (sear (meat lsb hib &) quad)
+          ==
+        ==
+      --
     ::                                                  ::  ++wish:de-json:html
     ++  wish                                            ::  with whitespace
       |*(sef=rule ;~(pfix spac sef))