Move the seeded randomness and the {seq()} counters into internal/drawstate

internal/drawstate/drawstate.go

@@ -0,0 +1,22 @@
+// Package drawstate holds what a generator draws through: the seeded randomness and the
+// {seq()} counters.
+package drawstate
+
+import "math/rand/v2"
+
+// State is one generator's draw state, kept across its renders.
+type State struct {
+	*rand.Rand
+	counters map[string]uint64
+}
+
+// New builds the state a seed reproduces.
+func New(seed uint64) *State {
+	return &State{Rand: rand.New(rand.NewPCG(seed, seed^0x9e3779b97f4a7c15)), counters: map[string]uint64{}}
+}
+
+// Next advances the counter key names, the one {seq()} or {seq(key)} reads, and returns it.
+func (s *State) Next(key string) uint64 {
+	s.counters[key]++
+	return s.counters[key]
+}