feat(v2): add fall event regression engine

v2/internal/fall/policy.go

@@ -0,0 +1,37 @@
+package fall
+
+type policy struct {
+	suspectWindowSeconds float64
+	requireRapidDrop     bool
+	rapidDropAt          map[string]float64
+}
+
+func newPolicy(suspectWindowSeconds float64, requireRapidDrop bool) *policy {
+	return &policy{
+		suspectWindowSeconds: suspectWindowSeconds,
+		requireRapidDrop:     requireRapidDrop,
+		rapidDropAt:          make(map[string]float64),
+	}
+}
+
+func (policy *policy) evaluate(trackID string, poseEvidence PoseEvidence, now float64, state State) Evidence {
+	if !poseEvidence.Accepted {
+		delete(policy.rapidDropAt, trackID)
+		return Evidence{}
+	}
+	if poseEvidence.RapidVerticalChange {
+		policy.rapidDropAt[trackID] = now
+	}
+	candidate := false
+	if state == Suspect {
+		candidate = poseEvidence.HorizontalPose
+	} else if poseEvidence.HorizontalPose {
+		if !policy.requireRapidDrop {
+			candidate = true
+		} else if droppedAt, found := policy.rapidDropAt[trackID]; found {
+			candidate = now-droppedAt <= policy.suspectWindowSeconds
+		}
+	}
+	recovery := (state == Confirmed || state == Recovering) && !poseEvidence.HorizontalPose && !poseEvidence.RapidVerticalChange
+	return Evidence{Accepted: true, IsFallCandidate: candidate, IsRecoveryCandidate: recovery}
+}