docs: add dartdoc comments to core public API classes

The generated `lib/bdk.dart` file exposes the full BDK API surface to Dart consumers but currently has no dartdoc comments on any of its public classes or methods. This means:

- `dart doc` produces empty or near-empty documentation pages
- pub.dev shows no API docs, which affects package scoring
- Developers have no in-editor guidance on usage, disposal, thread safety, or error handling

At a minimum, the following classes should have documented:
- `Wallet` — lifecycle, persistence, and disposal expectations
- `Descriptor` — construction patterns and supported script types
- `Mnemonic` — entropy, wordcount, and language options
- `ElectrumClient` / `EsploraClient` — connection setup and async behaviour
- `Transaction` / `Psbt` — building, signing, and broadcasting flow
- Exception types — there are 360+ exception classes; at least the common ones should explain when they're thrown

Since `lib/bdk.dart` is generated by UniFFI, the docs may need to be injected via UDL annotations or a post-generation script rather than edited directly.