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/

CLAUDE.md

@@ -0,0 +1,86 @@
+# firebase
+
+Firebase App component with launcher lifecycle and health check integration.
+
+## Purpose
+
+Manages the lifecycle of a `firebase.google.com/go/v4` App: validates config at init time,
+exposes the native `*fb.App` to consumers, and performs health checks by probing the Auth
+service. Does not wrap or abstract any Firebase service beyond App construction.
+
+## Tier & Dependencies
+
+**Tier 3 (infrastructure)** — depends on:
+- `code.nochebuena.dev/go/health` (Tier 2)
+- `code.nochebuena.dev/go/launcher` (Tier 2)
+- `code.nochebuena.dev/go/logz` (Tier 1)
+- `firebase.google.com/go/v4` (Firebase Admin SDK, wraps gRPC/HTTP internally)
+
+## Key Design Decisions
+
+- **Health check via known-nonexistent UID**: `HealthCheck` calls `GetUser` with the UID
+  `"health-probe-non-existent"` and uses `auth.IsUserNotFound(err)` to distinguish a
+  healthy "not found" response from a real connectivity failure. Avoids string matching
+  on error messages. See ADR-001.
+- **SDK lifecycle management**: `OnInit` calls `fb.NewApp`. `OnStop` is a no-op log
+  because the SDK has no `Close` method. See ADR-002.
+- **App exposed directly**: `App() *fb.App` gives consumers the full SDK entry point.
+  They call `app.Auth(ctx)` or `app.Firestore(ctx)` themselves. The module does not
+  cache or pre-construct service-specific clients.
+- **Health priority is Critical**: `Priority()` returns `health.LevelCritical`. A Firebase
+  Auth outage is treated as a critical failure.
+- **Duck-typed Logger**: The internal `logger` field is typed as `logz.Logger`, the shared
+  interface from the `logz` module (ADR-001 global pattern).
+
+## Patterns
+
+**Lifecycle registration:**
+```go
+fb := firebase.New(logger, cfg)
+lc.Append(fb) // registers OnInit / OnStart / OnStop
+```
+
+**Obtaining service clients from consumers:**
+```go
+type authService struct {
+    provider firebase.Provider
+}
+
+func (s *authService) VerifyToken(ctx context.Context, token string) (*auth.Token, error) {
+    authClient, err := s.provider.App().Auth(ctx)
+    if err != nil {
+        return nil, err
+    }
+    return authClient.VerifyIDToken(ctx, token)
+}
+```
+
+**Health check registration:**
+```go
+health.Register(fbComponent) // satisfies health.Checkable via Name()/Priority()/HealthCheck()
+```
+
+## What to Avoid
+
+- Do not call `App()` before `OnInit` has run — it returns `nil`. Guard against this in
+  tests or when constructing components manually outside a lifecycle manager.
+- Do not add service-specific methods (e.g. `AuthClient()`, `FirestoreClient()`) to this
+  module. Callers obtain them from `App()` directly, which keeps the module free of service
+  proliferation.
+- Do not increase the health check polling frequency significantly. Each `HealthCheck` call
+  makes a live API call to Firebase Auth. Poll at most every 30 seconds.
+- Do not rely on `OnStop` for graceful in-flight request drain. The SDK has no explicit
+  close; in-flight Firebase calls at shutdown must be handled by request context deadlines
+  in the caller.
+
+## Testing Notes
+
+- Unit tests do not require a real Firebase project. They test structural invariants:
+  `Name()`, `Priority()`, nil-safety of `OnStop`, and error on empty `ProjectID`.
+- `TestComponent_OnInit_MissingProjectID` verifies that `OnInit` returns an error before
+  attempting SDK initialisation when `ProjectID` is empty.
+- `TestComponent_App_ReturnsNilBeforeInit` confirms `App()` is nil before `OnInit` is called.
+- Integration tests (real token verification, real health check) require Firebase credentials
+  and belong outside this module.
+- `compliance_test.go` (package `firebase_test`) asserts `New(...)` satisfies `Component`
+  at compile time.