feat(firebase)!: promote to v1.0.0 — cache auth.Client, adopt xerrors, bump deps to v1

Cache auth.Client in OnInit (eliminates per-probe app.Auth call in HealthCheck).
Replace all fmt.Errorf with xerrors structured errors (ErrInvalidInput / ErrInternal).
Bump health/launcher/logz to v1.0.1, add xerrors v1.0.1, Go directive to 1.26.
API committed as stable.

CHANGELOG.md

@@ -5,6 +5,29 @@ All notable changes to this module will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this module adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.0.0] - 2026-05-12
+
+### Added
+
+- `authClient *auth.Client` cached in `OnInit` — the Auth client is now initialised once
+  during startup and reused in `HealthCheck`, eliminating a per-probe `app.Auth(ctx)` call.
+
+### Changed
+
+- Go directive bumped from 1.25 to 1.26
+- `health`, `launcher`, and `logz` dependencies bumped to v1.0.1
+- `xerrors v1.0.1` added — all error returns now use structured `xerrors.Err` instead of
+  `fmt.Errorf`; `OnInit` returns `ErrInvalidInput` for missing `ProjectID` and
+  `ErrInternal` for SDK initialisation failures; `HealthCheck` returns `ErrInternal` when
+  called before `OnInit`
+
+### Stabilization
+
+- API committed as stable. `Config`, `Provider`, `Component`, and `New` are unchanged
+  from v0.9.0.
+
+[1.0.0]: https://code.nochebuena.dev/go/firebase/compare/v0.9.0...v1.0.0
+
 ## [0.9.0] - 2026-03-18
 
 ### Added