Clarify config layer iteration APIs (#36774)

## What changed

- Replace the ordering enum and `include_disabled` boolean with named
  `ConfigLayerStack` iterators for each precedence direction.
- Keep enabled-only iteration separate from `all_layers_*` iteration, which
  includes disabled layers.
- Update config consumers to use the iterator matching their precedence and
  disabled-layer requirements without allocating an intermediate `Vec`.

## Testing

- Add coverage that verifies ordering and disabled-layer filtering for all four
  iterators.

GitOrigin-RevId: d14df3db4a2eae80ba97cdec246bff405e6c5f3d

codex-rs/tui/src/chatwidget/status_surfaces.rs

@@ -448,11 +448,7 @@ impl ChatWidget {
 
         self.config
             .config_layer_stack
-            .get_layers(
-                ConfigLayerStackOrdering::LowestPrecedenceFirst,
-                /*include_disabled*/ true,
-            )
-            .iter()
+            .all_layers_low_to_high()
             .find_map(|layer| match &layer.name {
                 ConfigLayerSource::Project { dot_codex_folder } => {
                     dot_codex_folder.as_path().parent().map(Path::to_path_buf)