Drop json-ng direct dependency in favor of stdlib json

json-ng has been absorbed into the pony stdlib as the `json` package
with an identical API. Since `use "json"` now resolves to stdlib,
we no longer need the explicit corral dependency.

Author: SeanTAllen

CLAUDE.md

@@ -23,7 +23,6 @@ Uses `corral` for dependency management. `make` automatically runs `corral fetch
 - `github.com/ponylang/lori.git` (via courier) -- TCP connections
 - `github.com/ponylang/ssl.git` (via courier) -- SSL/TLS
 - `github.com/ponylang/web_link.git` -- RFC 8288 Link header parsing
-- `github.com/ponylang/json-ng.git` -- JSON parsing (immutable, persistent collections)
 - `github.com/ponylang/uri.git` -- RFC 6570 URI template expansion
 
 ## Source Layout

corral.json

@@ -12,10 +12,6 @@
       "locator": "github.com/ponylang/courier.git",
       "version": "0.1.0"
     },
-    {
-      "locator": "github.com/ponylang/json-ng.git",
-      "version": "0.3.0"
-    },
     {
       "locator": "github.com/ponylang/uri.git",
       "version": "0.1.0"

github_rest_api/json_nav_util.pony

@@ -4,11 +4,10 @@ primitive JsonNavUtil
   """
   Utility for extracting optional string fields from JSON.
 
-  json-ng's JsonNav does not have an as_string_or_none() method. This
-  primitive provides equivalent functionality: given a JsonNav positioned on a
-  field, it returns the String value if present or None if the JSON value is
-  null. Raises an error if the navigation failed (key missing) or the value is
-  any other type.
+  JsonNav does not have an as_string_or_none() method. This primitive provides
+  equivalent functionality: given a JsonNav positioned on a field, it returns
+  the String value if present or None if the JSON value is null. Raises an
+  error if the navigation failed (key missing) or the value is any other type.
   """
   fun string_or_none(json: JsonNav): (String | None) ? =>
     match json.json()