H-17: the node-engine's own name is read from its applied record or first composing

Discovery, the finding writer and the vault login names read it; no fixed name.

Builds: design 47 G §527, §634 (A-149)

internal/selfcompose/selfcompose.go

@@ -160,7 +160,10 @@ const VaultSecretChanged = "mesh.mod.mesh-vault.event.secret-changed"
 
 // Composer composes the node and is the agent's watcher of its compositions (agent.Env.Composed).
 type Composer struct {
-	env Env
+	// OnEngine is told the node-engine's own module each time a composing finds it (design 47 G §527, §634); set before the
+	// composer runs.
+	OnEngine func(module string)
+	env      Env
 
 	mu          sync.Mutex
 	wants       map[string]meshwants.WantsByWantNodeModuleValue // by module, the node's own under agent.NodeModule
@@ -2453,7 +2456,11 @@ func (c *Composer) noteEngine(module string) {
 	}
 	c.mu.Lock()
 	c.engine = module
+	on := c.OnEngine
 	c.mu.Unlock()
+	if on != nil {
+		on(module)
+	}
 }
 
 // EngineModule is the node-engine's own module (design 47 G §527), for the apply guard; "" until a composing found it.