refactor: rename DTree to DValue

site/doc/pages/coming_from_react.txt

@@ -12,7 +12,7 @@ unit_render
 3_C++ Preprocessor
 map
 filter
-dtree_filter
+dv_filter
 
 :content
 UCE is server-first C++ with a small template preprocessor. It does not try to be React, but several concepts map cleanly.
@@ -47,8 +47,8 @@ The function library includes small collection helpers for common route/menu/car
 ```cpp
 auto visible = filter(routes, [](String route) { return(route != "admin"); });
 auto labels = map(visible, [](String route) { return(to_upper(route)); });
-DTree app_items = dtree_filter(menu, [](DTree item, String key) { return(item["section"].to_string() == "app"); });
-DTree by_section = dtree_group_by(menu, [](DTree item, String key) { return(item["section"].to_string()); });
+DValue app_items = dv_filter(menu, [](DValue item, String key) { return(item["section"].to_string() == "app"); });
+DValue by_section = dv_group_by(menu, [](DValue item, String key) { return(item["section"].to_string()); });
 ```
 
 Use these when the transformation communicates intent. Prefer explicit loops when side effects or multi-step validation are the main concern.