Rewrite for PHP 8.3, add Firestore support

[adrorocker]

Version 0.4 has not been installable for years. It required PHP 5.6 and Guzzle 6,
authenticated with a database secret in the query string, and called
GuzzleHttp\Psr7\stream_for(), which was removed in guzzle/psr7 2.0. A fresh
composer require could not boot. One repository also passed verify => false,
disabling TLS certificate verification.

This rewrites it for PHP 8.3+ and keeps the part that was worth keeping.

What changed

The bundled HTTP client is gone. It was never the valuable part, and
kreait/firebase-php does transport and credentials properly. What stays is the
entity and repository layer, rebuilt on top of a small Connection port.

Two adapters ship:

• Realtime Database, through kreait/firebase-php
• Firestore, over its REST API - no ext-grpc, so it runs on shared hosting

The entity layer knows nothing about Firebase, so the same entities,
repositories and queries run against either.

Notable

• Records map to typed objects: backed enums, dates, nested entities and plain
  value objects. Reading goes through cuyz/valinor; writing does not, because
  private properties and nulls have to stay out of the payload.
• Optimistic transactions with modify() and transaction(). Verified with real
  parallel processes rather than a mock: 4 writers, 20 increments, no lost
  updates on either backend.
• Batched writes - a multi-path update on RTDB, a batched commit on Firestore.
• Sentinel::ServerTimestamp and Sentinel::Remove.
• Queries the Realtime Database cannot run throw UnsupportedQuery instead of
  quietly answering a different question.

Breaking

Everything. PhpFirebase\Firebase, the Guzzle client and the extra/ autoload
path are removed, namespaces moved, and repositories now take a Connection.
See UPGRADING.md.

Realtime Database users need to require kreait/firebase-php explicitly - it is
not a hard dependency, since Firestore users do not need it.

Not included

Streaming. RTDB can push changes over text/event-stream and PHP can consume
them given a long-running runtime, but holding a connection open and reconnecting
it is infrastructure. This package maps entities.

Checks

165 tests, PHPStan level 8, php-cs-fixer. CI runs on 8.3, 8.4 and 8.5.