feat(presence): flood-fill presence mode

Add PresenceMode::flood alongside the default track_extent. In flood mode
the result sink snaps each presence claim to the shot it sits in, so an
actor seen once anywhere in a shot is reported for the whole shot
[prev_boundary, next_boundary]. This trades precision for recall against
X-Ray's per-scene cast granularity and is a toggleable knob for the
optimizer to weigh rather than a default.

Boundaries come from the frame stream, now carried through SceneAnnotation
(is_cut and is_scene_boundary). Flood prefers TransNetV2 shot boundaries
when a scene detector populated them, otherwise falls back to the
always-on histogram cuts (camera_position_change_detector); with no
boundaries it degrades to track_extent per claim. The is_scene_boundary
path stays dormant so an out-of-process scene detector can be revived
later without re-wiring.

Selected with --presence-mode flood|track_extent (default track_extent),
so existing output is byte-for-byte unchanged. The dump_embeddings header
note records why TransNetV2 scene detection is not run in that process.

src/nodes/frame_annotation_node.hpp

@@ -38,6 +38,11 @@ struct FrameAnnotationFunc {
 
     SceneAnnotation operator()(MatchedSceneFrame mf) {
         if (mf.source.eof) return {0.0, {}, /*eof=*/true};
-        return {mf.source.timestamp_sec, std::move(mf.actors)};
+        SceneAnnotation sa;
+        sa.timestamp_sec     = mf.source.timestamp_sec;
+        sa.visible_actors    = std::move(mf.actors);
+        sa.is_cut            = mf.source.is_cut;
+        sa.is_scene_boundary = mf.source.is_scene_boundary;
+        return sa;
     }
 };