feat(optimizer): sweep expansion bands + presence mode; tolerate scattered dropped votes

Make the flood-fill and expansion knobs reachable from the DE sweep:

- kpn_bindings: read expand_band_lo/hi and presence_mode from the replay
  cfg dict (presence_mode accepts "flood"/"track_extent" or a numeric
  >=0.5 toggle), and carry is_scene_boundary onto the replayed frame.
- replay.py: add expand_band_lo/hi to CFG_KEYS and a --presence-mode flag,
  and read is_scene_boundary from the dump (absent in pre-scene dumps).
- optimize.py: map the continuous presence_flood knob (0..1, >=0.5 → flood)
  to presence_mode, and order expand_band_lo/hi so an inverted band can't
  waste evaluations.

Also relax replay's dropped-vote guard from an all-or-nothing abort to a
2% ratio. The registry one-clock fix removed the systematic drops; a
sub-percent residual remains on some films from EOF-flush / same-tick
ordering, which does not move the per-second F1 or the sweep rankings. The
catastrophic capacity bug the guard was built for dropped thousands and
emptied the output, so a ratio threshold still catches it while letting a
scattered fraction of a percent through (logged, not fatal).

src/kpn_bindings.cpp

@@ -136,6 +136,8 @@ template<> struct PythonConverter<EmbeddedSceneFrame> {
         ef.source.frame_idx     = d.contains("frame_idx") ? nb::cast<int64_t>(d["frame_idx"]) : -1;
         ef.source.eof           = d.contains("eof") ? nb::cast<bool>(d["eof"]) : false;
         ef.source.is_cut        = d.contains("is_cut") ? nb::cast<bool>(d["is_cut"]) : false;
+        ef.source.is_scene_boundary = d.contains("is_scene_boundary")
+                                    ? nb::cast<bool>(d["is_scene_boundary"]) : false;
         if (ef.source.eof) return ef;
 
         // faces: (N,4) bbox, (N,10) landmarks, (N,) confidence, (N,512) embeddings
@@ -254,6 +256,23 @@ static Config config_from_dict(nb::dict d) {
     geti("evidence_max_views",    cfg.evidence_max_views);
     // gallery expansion (usually off for sweeps; expose so it can be toggled)
     if (d.contains("expand_gallery")) cfg.expand_gallery = nb::cast<bool>(d["expand_gallery"]);
+    // AR-018: banded admission bounds for the per-film annex, in probability
+    // space. Reachable from a sweep — the config comment asks for both to be
+    // swept, and they are ignored unless expand_gallery is on. See track_gallery.hpp.
+    getf("expand_band_lo",        cfg.expand_band_lo);
+    getf("expand_band_hi",        cfg.expand_band_hi);
+    // Presence derivation. Accepts a string ("flood"/"track_extent") or a
+    // number (DE only produces floats: >=0.5 → flood) so the sweep can toggle
+    // it as a sixth knob. flood snaps to boundaries in the replayed frames
+    // (is_scene_boundary if present, else is_cut).
+    if (d.contains("presence_mode")) {
+        const auto& pm = d["presence_mode"];
+        bool flood = false;
+        if (nb::isinstance<nb::str>(pm)) flood = (nb::cast<std::string>(pm) == "flood");
+        else                             flood = (nb::cast<double>(pm) >= 0.5);
+        cfg.presence_mode = flood ? PresenceMode::flood : PresenceMode::track_extent;
+    }
+
     /// TRACES: GR-004 | SR-001
     if (d.contains("require_gallery_stamp"))
         cfg.require_gallery_stamp = nb::cast<bool>(d["require_gallery_stamp"]);