Compose a node's plans once per read, resolving each machine once (issue 1954)

A node's plans took 2.6-13.6 s of CPU on the provisioner: one composition
resolved every machine of the mesh some two hundred times (who is on the
private network, and the rest of the mesh, worked out again on every
look), three compositions did so again each, and the holder composed
twice. A composer now works each out once over its unchanging Input;
Composing gives Resolve, Render and Generators one composer;
PlansHeldTwice composes the wanted and the newest builds over one read of
the bus, says where the time went by phase, and the states reader keeps a
build's take-in by its content. WatchedStates takes a process's own
writes into the copy at once, so a holder reads its own states from it.

Measured on a captured novox mesh: 11.7 s to 0.49 s per composition of an
unchanged node, plans and provisions identical on all four nodes.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

go/compose/render.go

@@ -35,6 +35,26 @@ func Render(in Input, node string, mode Mode) (Rendered, error) {
 	return newComposer(in).render(node, mode)
 }
 
+// Composing is Resolve, Render and Generators of one node over one composition, which resolves each node of the mesh
+// once for all three (issue 1954: three compositions resolved the mesh again each, 11.7 s for one node's plans). Each
+// answers as it does alone; the first refusal ends it, in that order, with what follows it empty.
+func Composing(in Input, node string, mode Mode) (Resolved, Rendered, map[string]catalogue.Generator, error) {
+	c := newComposer(in)
+	resolved, err := c.planFor(node)
+	if err != nil {
+		return Resolved{}, Rendered{}, nil, err
+	}
+	rendered, err := c.render(node, mode)
+	if err != nil {
+		return resolved, Rendered{}, nil, err
+	}
+	gens, err := c.generators()
+	if err != nil {
+		return resolved, rendered, nil, err
+	}
+	return resolved, rendered, gens, nil
+}
+
 func (c *composer) render(nodeName string, mode Mode) (Rendered, error) {
 	resolved, err := c.planFor(nodeName)
 	if err != nil {