refactor: EtlPipeline entry via Create() instance seed (Option 2)

[Chris-Wolfgang]

Summary

Switches the EtlPipeline entry point from the static-class + Source sentinel design to the instance seed design we settled on (Option 2), so it can ship with 0.16.0.

// before (Option 1 — sentinel)
await EtlPipeline.From(records).Through(...).To(loader).RunAsync();
await EtlPipeline.Source.CsvExtractor<Order>("orders.csv")...   // format factory on the sentinel

// after (Option 2 — Create() seed)
await EtlPipeline.Create().From(records).Through(...).To(loader).RunAsync();
await EtlPipeline.Create().CsvExtractor<Order>("orders.csv")... // format factory on the instance

Changes

• EtlPipeline: public static class → public sealed class with a private ctor and static Create(); the From factories become instance methods.
• Removed EtlPipelineSource — the EtlPipeline instance is now the extension receiver for format-package source factories (this EtlPipeline), so the separate sentinel is obsolete.
• Updated PublicAPI.Shipped.txt, Example8-EtlPipeline (net4.8 + net8.0), the EtlPipeline unit tests, and the CHANGELOG 0.16.0 entry.

Unchanged

The pipeline core is untouched — IEtlPipeline<T> (Through/To/AsAsyncEnumerable), IEtlPipelineSink, EtlPipelineImpl, EtlPipelineProgress, run-state. Only the entry point changed.

Why

Create() returning a fresh instance reads honestly as "start a pipeline," removes the .Source naming/asymmetry wart, keeps the unified EtlPipeline prefix, and gives pipeline-level config a natural home on the seed if ever needed. One type instead of two.

Test plan

• dotnet build -c Release clean across all 11 TFMs
• Unit tests green (277) — From/Through/To/cancellation/progress + new Create_returns_a_new_instance_each_call
• Example8 (net4.8 + net8.0) compiles against Create()
• CI Stage 1/2/3 + Detect .NET Projects green

Note on operators

Per the core's own design (and this PR's docs), the LINQ operators (Where/Select/Distinct) live in Wolfgang.Etl.Transformers as extensions on Through, not here — out of scope for this entry-point change.

🤖 Generated with Claude Code

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.