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/config.hpp

@@ -11,6 +11,20 @@ enum class Verbosity {
     standard,  // per-frame detail: bbox, similarity, unknowns logged
     xray,      // Jellyfin-Xray format: {"second": ["Actor", ...], ...}
 };
+
+// How a track's accepted frames become a reported presence window.
+enum class PresenceMode {
+    // A claim IS its track's [first_seen, last_seen] (AR-012/AR-013). The
+    // default and the only mode whose semantics the register validated.
+    track_extent,
+    // Flood-fill: snap each claim to the shot it sits in, so an actor seen once
+    // anywhere in a scene is reported for the whole scene [prev_boundary,
+    // next_boundary]. Trades precision for recall against X-Ray's per-scene cast
+    // granularity. Snaps to TransNetV2 shot boundaries (is_scene_boundary) when a
+    // scene detector populated them, else to the always-on histogram cuts
+    // (is_cut). With no boundaries at all it degrades to track_extent per claim.
+    flood,
+};
 // debug verbosity = compile with -DSAE_DEBUG → scene_analyze_debug binary
 
 struct Config {
@@ -115,6 +129,10 @@ struct Config {
     // fallback everywhere, which is at least the same wrong number in every
     // stage. See identity_matcher_node.hpp.
 
+    // ── Presence derivation ──────────────────────────────────────────────────
+    // How accepted frames become a reported window. flood requires scene_detect.
+    PresenceMode presence_mode{PresenceMode::track_extent};
+
     // ── Cut detection ────────────────────────────────────────────────────────
     float cut_threshold{0.70f};         // grayscale histogram correlation below this → hard cut