[sacOO7]

Summary

Restructures the README for the 0.3.0 ably_pubsub_device package to match the other Ably Pub/Sub
2.0 READMEs: an overview modelled on the Go README, two-sentence requirements and the migration
table after the quick start. Only README.md changes.

What changed

• Overview: an experimental-API callout, the device-only note and a table for ably_pubsub_device and createClient.
• Requirements: Dart 3.0 or later, on every platform that provides dart:io.
• Installation: dart pub add ably_pubsub_device, or flutter pub add in Flutter apps.
• Quick start: an authUrl sample. authCallback is one sentence.
• Migrating from 0.2.x: a top-level section after Quick start, with a 0.2.x to 0.3.x table and a link to UPGRADING.md.
• Common configuration: Endpoint, with the port and TLS rules, and Logging.
• Known limitations: no web support and no push device activation.
• API key: no sample uses one. The device authenticates with a token only.
• Links: repository files use absolute GitHub URLs.

How it was validated

• Every claim was checked against the source on main: createClient, ClientOptions, the AuthCallback return types, LogLevel, pubspec.yaml and UPGRADING.md.
• All Dart snippets pass dart analyze in a scratch package. A web build fails as documented.
• Links and badges return 200. The UPGRADING.md anchor exists at main.
• An independent review checked facts, samples, links and markdown rendering.

Risk Assessment

[ttypic]

LGTM

[ttypic]

LGTM