Add fluent Pipeline API for Extract -> Transform -> Load (#83)

[Chris-Wolfgang]

Summary

Implements the fluent Pipeline API agreed on in #83 (see design-decisions comment).

await Pipeline
    .Extract(extractor).WithProgress(extractProgress)
    .Transform(t1)
    .Transform(t2).WithProgress(t2Progress)
    .Load(loader).WithProgress(loadProgress)
    .WithName("nightly-import")
    .RunAsync(cancellationToken);

What's included

Public API (all in Wolfgang.Etl.Abstractions namespace):

• Pipeline — static entry with overloaded Extract<T> / Extract<T, TProgress>
• IExtractStage<T> / IExtractStageWithProgress<T, TProgress>
• ITransformStage<T> / ITransformStageWithProgress<T, TProgress>
• IPipeline / IPipelineWithLoadProgress<TProgress> — runnable terminals with WithName, RunAsync(), RunAsync(CancellationToken)

Key properties:

• Compile-time stage-to-stage type safety (mismatched stages are red squiggles, not runtime throws)
• .WithProgress(...) only appears on stage types whose underlying extractor/transformer/loader supports progress
• One-shot execution: a second RunAsync call throws InvalidOperationException
• Raw exception propagation (no wrapping)
• Pipeline owns nothing — caller disposes stages

Out of scope (per design)

• PipelineReport composite return type
• Auto-disposal of stages (tracked in Add opt-in .DisposeStagesOnCompletion() to Pipeline builder #133)
• Per-item middleware / Use(...) (depends on Add middleware / interceptor support for cross-cutting per-item concerns #93)
• Reusable / multi-run pipelines
• Parallel transformer execution
• Progress supplied at RunAsync call time

Tests

19 new tests covering:

• Null-argument guards on every factory, stage method, and setter
• Simple E -> L, E -> T -> L, and multi-transformer E -> T -> T -> T -> L composition (with type-changing transformers)
• WithProgress forwarding on extractor, transformer, and loader
• WithName default and assignment
• One-shot enforcement on both pipeline variants
• Cancellation token forwarded to stages
• Raw exception propagation
• Stage state observable after a mid-stream failure

All 151 unit tests pass in Release on net10.0.

Test plan

• dotnet build -c Release -f net10.0 clean (0 warnings, 0 errors) on src + tests
• dotnet test -c Release -f net10.0 — 151/151 pass
• CI matrix builds across all TFMs

Closes #83.

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