Move a table's rows, links and pins into internal/rows, and walk every path through one per-step function, so Fake refuses a level carrying a repeat

internal/drawstate/drawstate.go

@@ -1,7 +1,10 @@
 // Package drawstate holds a generator's seeded randomness and its {seq()} counters.
 package drawstate
 
-import "math/rand/v2"
+import (
+	"math/rand/v2"
+	"sort"
+)
 
 // State is one generator's draw state, kept across its renders.
 type State struct {
@@ -18,6 +21,12 @@ func (s *State) IntN(n int) int { return s.rand.IntN(n) }
 
 func (s *State) Float64() float64 { return s.rand.Float64() }
 
+// Weighted draws an index into cum, a running sum of positive weights, with odds by weight.
+func (s *State) Weighted(cum []float64) int {
+	x := s.Float64() * cum[len(cum)-1]
+	return min(sort.Search(len(cum), func(i int) bool { return cum[i] > x }), len(cum)-1) // x can round up to the total
+}
+
 // Seq advances the counter {seq(key)} reads, or {seq()} for an empty key, and returns it.
 func (s *State) Seq(key string) uint64 {
 	s.counters[key]++