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.