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/dump_embeddings.cpp

@@ -8,6 +8,12 @@
 // gallery file needed. Purpose-built for the optimizer's replay corpus and the
 // embedding-model bake-off (dump each --arcface model over the film set).
 //
+// The camera-position (histogram) cut detector runs, so frames/is_cut is recorded
+// in the dump. Flood-fill presence snaps to those cuts. TransNetV2 scene detection
+// is NOT run here: on the ROCm build it needs MIGraphX, whose statically-linked
+// LLVM collides with the VAAPI decoder's system libLLVM and crashes at compile, so
+// the two cannot share a process. is_scene_boundary therefore stays 0 here.
+//
 // Usage:
 //   dump_embeddings --movie <path> --out <dump.h5> [--arcface <model.onnx>]
 //                   [--detector <model.onnx>] [--fps 1] [--start S] [--end S]