feat(01-02): add optional plugin services and HasPlugin

- App-scoped typed Publish/Lookup with duplicate-provider errors
- Set plugin IDs before Register so HasPlugin sees the full set
- Greeter uses pact.OptionalMessage without importing optional

backpack/app.go

@@ -1,13 +1,71 @@
 package backpack
 
-import "git.golem15.com/golem15/summercms/compass"
+import (
+	"fmt"
+	"sync"
+
+	"git.golem15.com/golem15/summercms/compass"
+)
 
 // App is the per-instance application container. It must not import party.
 type App struct {
-	Config *compass.Config
+	Config   *compass.Config
+	Services *Registry
+
+	mu      sync.RWMutex
+	plugins map[string]struct{}
 }
 
 // New returns an App that owns cfg.
 func New(cfg *compass.Config) *App {
-	return &App{Config: cfg}
+	return &App{
+		Config:   cfg,
+		Services: NewRegistry(),
+		plugins:  make(map[string]struct{}),
+	}
+}
+
+// SetPlugins records the complete activated plugin set. HasPlugin uses this
+// list, including plugins that have not yet Booted.
+func (a *App) SetPlugins(ids []string) {
+	if a == nil {
+		return
+	}
+	next := make(map[string]struct{}, len(ids))
+	for _, id := range ids {
+		if id != "" {
+			next[id] = struct{}{}
+		}
+	}
+	a.mu.Lock()
+	a.plugins = next
+	a.mu.Unlock()
+}
+
+// HasPlugin reports whether id is in the complete registered set.
+func (a *App) HasPlugin(id string) bool {
+	if a == nil {
+		return false
+	}
+	a.mu.RLock()
+	defer a.mu.RUnlock()
+	_, ok := a.plugins[id]
+	return ok
+}
+
+// Publish stores an app-scoped service under T.
+func (a *App) Publish[T any](value T) error {
+	if a == nil || a.Services == nil {
+		return fmt.Errorf("backpack: app has no service registry")
+	}
+	return a.Services.Publish(value)
+}
+
+// Lookup returns the app-scoped service published under T.
+func (a *App) Lookup[T any]() (T, bool) {
+	var zero T
+	if a == nil || a.Services == nil {
+		return zero, false
+	}
+	return a.Services.Lookup[T]()
 }