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,

semantic/llm_client.py

@@ -7,14 +7,6 @@ from semantic.config import DEFAULT_MODEL
 from semantic.models import SEMANTIC_SCENE_SCHEMA, SemanticScene
 from semantic.prompts import ENRICHMENT_SYSTEM_PROMPT, scene_user_prompt
 
-_SDK_UNAVAILABLE_EXCEPTION_NAMES = {
-    "APITimeoutError",
-    "APIConnectionError",
-    "RateLimitError",
-    "AuthenticationError",
-    "APIStatusError",
-}
-
 
 class EnrichmentUnavailable(Exception):
     """Internal signal for expected enrichment transport/response failures."""

tests/test_semantic_enricher.py

@@ -123,7 +123,7 @@ def test_enrich_scene_filters_dangling_element_ids() -> None:
     ]
 
 
-def test_enrich_scene_returns_none_when_all_widgets_are_dangling() -> None:
+def test_enrich_scene_returns_empty_widgets_when_all_widgets_are_dangling() -> None:
     client = FakeClient(
         response={
             "page": "Chat",
@@ -138,4 +138,7 @@ def test_enrich_scene_returns_none_when_all_widgets_are_dangling() -> None:
         config=SemanticConfig(enabled=True),
     )
 
-    assert result is None
+    assert result is not None
+    assert result.page == "Chat"
+    assert result.intents == ["send a message"]
+    assert result.widgets == []