docs: add CHANGELOG.md with 0.4.0 release notes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jay-cox

CHANGELOG.md

@@ -0,0 +1,19 @@
+# Changelog
+
+## 0.4.0
+
+### Breaking Changes
+
+- **Firebase v12 modular API migration**: Replaced all `firebase/compat/*` imports with modular equivalents from `firebase/app`, `firebase/database`, and `firebase/auth`
+- `signOut` is now an instance method instead of a static method (requires an `Auth` instance)
+- `peerDependencies.firebase` updated from `^9.8.3` to `^12.0.0`
+
+### Fixed
+
+- `collectionObserver`: `limitToFirst` constraint now correctly references `props.limitToFirst` instead of an undefined local variable
+
+### Changed
+
+- TypeScript compilation target updated from ES2015 to ES2020
+- CI workflows updated to Node.js 20 and actions v4
+- Removed `firebase-init.ts` compat singleton wrapper