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/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."""