feat(firebase): initial stable release v0.9.0

Firebase App component with launcher lifecycle and health check integration.

What's included:
- Config with ProjectID (FIREBASE_PROJECT_ID env var); credentials via ADC
- Provider interface exposing native *firebase.App directly
- Component interface: launcher.Component + health.Checkable + Provider
- New(logger, cfg) constructor for lifecycle registration via lc.Append
- Health check via GetUser("health-probe-non-existent") + auth.IsUserNotFound at LevelCritical
- No-op OnStop (Firebase Admin SDK has no Close method)

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

firebase_test.go

@@ -0,0 +1,51 @@
+package firebase
+
+import (
+	"testing"
+
+	"code.nochebuena.dev/go/health"
+	"code.nochebuena.dev/go/logz"
+)
+
+func newLogger() logz.Logger { return logz.New(logz.Options{}) }
+
+func TestNew(t *testing.T) {
+	if New(newLogger(), Config{ProjectID: "test"}) == nil {
+		t.Fatal("New returned nil")
+	}
+}
+
+func TestComponent_Name(t *testing.T) {
+	c := New(newLogger(), Config{ProjectID: "test"}).(health.Checkable)
+	if c.Name() != "firebase" {
+		t.Errorf("want firebase, got %s", c.Name())
+	}
+}
+
+func TestComponent_Priority(t *testing.T) {
+	c := New(newLogger(), Config{ProjectID: "test"}).(health.Checkable)
+	if c.Priority() != health.LevelCritical {
+		t.Error("Priority() != LevelCritical")
+	}
+}
+
+func TestComponent_OnInit_MissingProjectID(t *testing.T) {
+	c := New(newLogger(), Config{ProjectID: ""})
+	if err := c.OnInit(); err == nil {
+		t.Error("expected error for empty ProjectID")
+	}
+}
+
+func TestComponent_OnStop_NilApp(t *testing.T) {
+	c := &firebaseComponent{logger: newLogger()}
+	if err := c.OnStop(); err != nil {
+		t.Errorf("OnStop with nil app: %v", err)
+	}
+}
+
+func TestComponent_App_ReturnsNilBeforeInit(t *testing.T) {
+	c := New(newLogger(), Config{ProjectID: "test"})
+	if c.App() != nil {
+		t.Error("App() should be nil before OnInit")
+	}
+}