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/types.hpp

@@ -155,6 +155,13 @@ struct SceneAnnotation {
     double                       timestamp_sec{0.0};
     std::vector<IdentifiedActor> visible_actors;
     bool                         eof{false};
+    // Carried through from Frame so the sink can collect boundaries for flood-fill
+    // presence (PresenceMode::flood). is_cut is the always-on histogram cut
+    // (camera_position_change_detector) — the boundary flood-fill uses by default.
+    // is_scene_boundary is the opt-in TransNetV2 shot boundary (0 unless scene
+    // detection ran); kept for a future out-of-process scene detector.
+    bool                         is_cut{false};
+    bool                         is_scene_boundary{false};
 };
 
 // ── Actor gallery ─────────────────────────────────────────────────────────────