feat: move stuff to numerics

internal/engine/registry.go

@@ -241,25 +241,31 @@ func buildPiecewise(spec ModelSpec, deps BuildDeps) (BuiltModel, error) {
 	}, nil
 }
 
-// resolveSegments converts spec segments to engine.RateSegment using the
-// stage context to resolve relative references.
+// resolveSegments converts spec segments to engine.RateSegment, turning each
+// segment's reference-relative Until into an absolute UNIX time. References
+// are validated by buildPiecewise, so an unrecognised one here is treated as
+// absolute rather than re-erroring.
 func resolveSegments(in []PiecewiseSegmentSpec, ctx StageContext) []RateSegment {
 	out := make([]RateSegment, 0, len(in))
 	for _, s := range in {
-		var until float64
-		switch s.Reference {
-		case "", "absolute":
-			until = s.Until
-		case "profile_start":
-			until = ctx.ProfileStart + s.Until
-		case "propagator_start":
-			until = ctx.PropagatorStart + s.Until
-		}
-		out = append(out, RateSegment{Until: until, Rate: s.Rate})
+		out = append(out, RateSegment{Until: segmentBase(s.Reference, ctx) + s.Until, Rate: s.Rate})
 	}
 	return out
 }
 
+// segmentBase returns the absolute time a piecewise segment's Until is
+// measured from, per its reference.
+func segmentBase(reference string, ctx StageContext) float64 {
+	switch reference {
+	case "profile_start":
+		return ctx.ProfileStart
+	case "propagator_start":
+		return ctx.PropagatorStart
+	default: // "", "absolute"
+		return 0
+	}
+}
+
 // maybeAddWind sums a WindTransport model into base when the spec asks for it.
 func maybeAddWind(base Model, includeWind bool, deps BuildDeps) Model {
 	if !includeWind {