- _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
q792602257