feat: IAsyncDisposable/IDisposable on base classes (#92)

[Chris-Wolfgang]

Adds IAsyncDisposable + IDisposable to all three base classes. Closes #92.

Stacked on #247 (base feature/reset-run-state) — it's the tip of the base-class change chain (#244 → #247 → this).

What

ExtractorBase, LoaderBase, TransformerBase now implement both interfaces with the standard dispose pattern and a default no-op:

• virtual ValueTask DisposeAsync() and void Dispose() → Dispose(true) + GC.SuppressFinalize
• protected virtual void Dispose(bool disposing) — base just marks disposed (idempotent); derived classes override to release the resources they own (streams, DB connections, HTTP clients, …) and call base.Dispose(disposing)

await using var extractor = new DatabaseExtractor(connectionString);
await foreach (var row in extractor.ExtractAsync(token)) { }

Every component works with await using out of the box, and base-class stages are now disposable via the #133 DisposeStagesOnCompletion path — the two features compose.

Compatibility

MINOR, source-compatible but binary-breaking (adding interfaces to a base class requires consumers to recompile; subclasses that already implement IDisposable switch to the override keyword). Needs a call-out in the 0.14.0 release notes. IAsyncDisposable is available on all TFMs via the existing Microsoft.Bcl.AsyncInterfaces reference.

Verified

• Full Release build clean across all TFMs (0 warnings); PublicAPI.Shipped.txt regenerated.
• 239 tests pass (5 new: interfaces present, default no-op is safe, override invoked via Dispose() / DisposeAsync() / await using).

Part of: #154