suite rule и suite list

- suite rule дописывает правило: номер берётся следующим за наибольшим, блоки
  раскладываются в порядке норма, ПОЧЕМУ, ПРИМЕРЫ, --after ставит правило рядом
  с тем, которое оно уточняет
- ступень называется категорией (requirement, prohibition, ...), а не словом
  языка, поэтому вызывающему не нужно знать, на каком языке записан набор
- suite list показывает темы со слоями, а с осью — что возьмёт компонент; отбор
  слоёв вынесен в suite.Assemble, откуда его возьмут проектные команды
- слои темы теперь всегда возвращаются базовым вперёд

internal/suite/assemble.go

@@ -0,0 +1,71 @@
+package suite
+
+import "git.vakhrushev.me/av/convy/internal/doc"
+
+// Component is what a copy is assembled for: one language, one stack, one kind
+// of application. Both axes may stay empty — a flat suite has no axes at all,
+// and a component of it selects the base layer and nothing else.
+type Component struct {
+	Lang  string
+	Stack string
+}
+
+// Assemble picks the layers of a topic a component takes, in the order they go
+// into a copy: base, then language, then stack.
+//
+// A layer is chosen by the axis keys of its front matter rather than by where
+// its file lies (META-38). A layer with no keys is the base one and travels
+// always; a key the layer does not declare puts no demand on the component, so
+// a language layer with no stack key fits any stack.
+//
+// This is the read-only half of assembly. The project commands write files out
+// of the same selection, so the two must never come to differ — which is why
+// the selection lives here rather than inside whichever command needs it.
+func (s *Suite) Assemble(topic string, c Component) (taken, left []*doc.Document) {
+	for _, d := range s.Layers(topic) {
+		if fits(d, c) {
+			taken = append(taken, d)
+			continue
+		}
+		left = append(left, d)
+	}
+	return taken, left
+}
+
+// fits reports whether a component takes a layer.
+func fits(d *doc.Document, c Component) bool {
+	if d.Front.Lang != "" && d.Front.Lang != c.Lang {
+		return false
+	}
+	if d.Front.Stack != "" && d.Front.Stack != c.Stack {
+		return false
+	}
+	return true
+}
+
+// rank orders the layers of a topic the way a copy carries them. A layer on
+// both axes comes last: it narrows the most.
+func rank(d *doc.Document) int {
+	switch {
+	case d.Front.Lang == "" && d.Front.Stack == "":
+		return 0
+	case d.Front.Stack == "":
+		return 1
+	case d.Front.Lang == "":
+		return 2
+	}
+	return 3
+}
+
+// Axis describes a layer in one word, for a report.
+func Axis(d *doc.Document) string {
+	switch {
+	case d.Front.Lang != "" && d.Front.Stack != "":
+		return "lang=" + d.Front.Lang + " stack=" + d.Front.Stack
+	case d.Front.Lang != "":
+		return "lang=" + d.Front.Lang
+	case d.Front.Stack != "":
+		return "stack=" + d.Front.Stack
+	}
+	return "base"
+}