Add json_get_raw builtin to El VM

Adds json_get_raw(value, key) -> String that serializes any JSON field
(including nested objects and arrays) back to a JSON string. Existing
json_get_string only handles primitive string values. This enables
traversing nested JSON structures in El programs.

Also registers json_get_raw in is_builtin().

bin/el/src/main.rs

@@ -4187,6 +4187,45 @@ fn dispatch_builtin(
             BuiltinResult::Handled
         }
 
+        // ── JSON raw field extractor ──────────────────────────────────────────
+        // json_get_raw(json_str_or_struct, key) -> String
+        // Returns the JSON-serialized value at key, including objects and arrays.
+        // Unlike json_get_string which only handles primitive strings, this works
+        // for any JSON type and always returns a valid JSON string.
+        "json_get_raw" => {
+            let key = match stack.pop().unwrap_or(Value::Nil) {
+                Value::Str(s) => s,
+                _ => String::new(),
+            };
+            let v = stack.pop().unwrap_or(Value::Nil);
+            let result = match &v {
+                Value::Map(pairs) => pairs.iter()
+                    .find(|(k, _)| k == &key)
+                    .map(|(_, v)| {
+                        let jv = el_value_to_json_value(v);
+                        serde_json::to_string(&jv).unwrap_or_default()
+                    })
+                    .unwrap_or_default(),
+                Value::Struct { fields, .. } => fields.iter()
+                    .find(|(k, _)| k == &key)
+                    .map(|(_, v)| {
+                        let jv = el_value_to_json_value(v);
+                        serde_json::to_string(&jv).unwrap_or_default()
+                    })
+                    .unwrap_or_default(),
+                Value::Str(json_str) => {
+                    serde_json::from_str::<serde_json::Value>(json_str)
+                        .ok()
+                        .and_then(|jv| jv.get(&key).cloned())
+                        .map(|v| serde_json::to_string(&v).unwrap_or_default())
+                        .unwrap_or_default()
+                }
+                _ => String::new(),
+            };
+            stack.push(Value::Str(result));
+            BuiltinResult::Handled
+        }
+
         // ── HTTP auth builtins (Bearer token) ─────────────────────────────────
 
         "http_get_auth" => {
@@ -6521,7 +6560,7 @@ fn is_builtin(name: &str) -> bool {
         | "hash_sha256" | "blake3_hash"
         | "float_to_str" | "int_to_str" | "str_to_int" | "str_to_float" | "parse_float" | "parse_int" | "bool_to_str"
         | "bytes_to_str" | "str_to_bytes"
-        | "json_get" | "json_get_int" | "json_get_string" | "json_get_array" | "json_get_bool" | "json_get_float"
+        | "json_get" | "json_get_int" | "json_get_string" | "json_get_array" | "json_get_bool" | "json_get_float" | "json_get_raw"
         | "json_set" | "json_stringify" | "json_parse" | "json_encode" | "json_decode" | "json_keys"
         | "json_array_get" | "json_array_len" | "json_array_push"
         | "str_eq" | "str_split" | "str_contains" | "str_starts_with" | "str_ends_with" | "str_replace"