fix(semantic-scene): preserve page/intents when all widgets are dangling, drop dead exception-name set

- _with_known_widgets_only used to discard the entire SemanticScene
  (returning None) when filtering dangling-element-id widgets left an
  empty list, losing valid page/intents. Now returns a SemanticScene
  with widgets=[] instead, matching design.md's 'dropped, not a hard
  failure' intent.
- Removed _SDK_UNAVAILABLE_EXCEPTION_NAMES, dead code left over from an
  abandoned name-based exception-matching approach; the blanket except
  Exception already maps every SDK failure correctly.

openspec: semantic-scene capability, archived change semantic-scene-runtime

semantic/enricher.py

@@ -42,7 +42,7 @@ def enrich_scene(
 def _with_known_widgets_only(
     scene: Scene,
     semantic_scene: SemanticScene,
-) -> SemanticScene | None:
+) -> SemanticScene:
     known_element_ids = {element.id for element in scene.elements}
     if not known_element_ids:
         return semantic_scene
@@ -52,8 +52,6 @@ def _with_known_widgets_only(
         for widget in semantic_scene.widgets
         if widget.element_id in known_element_ids
     ]
-    if not widgets:
-        return None
 
     return SemanticScene(
         page=semantic_scene.page,