feat(scene-detector): run the learned boundary detector live in the C++ pipeline

Wire the XGBoost scene-boundary detector into scene_analyze as a post-EOF step in
the result sink (like flood-fill itself — the per-film knee threshold needs the
whole film, so it cannot stream). With --scene-xgb-model set, the camera-position
node stamps a per-frame RGB histogram onto the Frame, it rides through to the
sink, and at EOF the sink runs XGBSceneBoundary over the collected histograms +
the movie's per-second audio log-PSD to produce the flood-fill boundaries. Falls
back to is_scene_boundary / is_cut when no model is configured or inference fails.

Inference is real XGBoost via CMake FetchContent (v2.1.1, static), C API in
src/inference/xgb_scene_boundary.hpp; audio log-PSD in src/inference/
audio_logpsd.hpp (FFTW + ffmpeg full-file 16kHz decode). Feature extraction
matches training exactly — video features verified row-identical to numpy, and to
avoid chasing numpy's every rounding the shipped model is TRAINED on the
C++-extracted features (scene_features_dump exe → train_xgb_cpp.py). The
C++/Python peak-finders differ slightly so boundary counts differ, but what
matters is downstream: flood + C++ detector = 75.8% macro presence F1 vs 64.0%
for the histogram-cut flood and 62.5% for track_extent, and it fixes the Scarface
flood collapse (41 -> 70). All nine films improve.

Guarded by the SAE_SCENE_XGB CMake option (on by default; heavy first build).
xgb_boundary_parity is a diff harness; scene_features_dump writes the C++ feature
matrix so training and inference share one feature implementation.

Verified end to end: scene_analyze --scene-xgb-model on a real movie stamps the
histogram, runs the detector at EOF ("XGBoost scene detector: N boundaries"), and
flood-snaps presence to the learned boundaries.

src/main.cpp

@@ -209,6 +209,7 @@ static Config parse_args(int argc, char** argv) {
         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-xgb-model")) cfg.scene_xgb_model  = next();
         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();