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/main.cpp

@@ -208,6 +208,7 @@ static Config parse_args(int argc, char** argv) {
         else if (arg("--start"))           cfg.start_sec        = std::stod(next());
         else if (arg("--end"))             cfg.end_sec          = std::stod(next());
         else if (arg("--cut-threshold"))   cfg.cut_threshold    = std::stof(next());
+        else if (arg("--presence-mode"))   { std::string m = next(); cfg.presence_mode = (m == "flood") ? PresenceMode::flood : PresenceMode::track_extent; }
         else if (arg("--scene-detect"))         cfg.scene_detect     = true;
         else if (arg("--scene-detector"))       cfg.scene_model      = next();
         else if (arg("--scene-detector-engine")) cfg.scene_engine    = next();