Composable per-item middleware / interceptor (#93)

[Chris-Wolfgang]

Closes #93. Stacked on #333 (#94) — base is feat/94-retry-seam.

Summary

Adds a dependency-free middleware / interceptor mechanism for cross-cutting per-item concerns:

• IItemMiddleware<T> — ValueTask<MiddlewareResult<T>> OnItemAsync(T item, CancellationToken).
• MiddlewareResult<T> — value type with Item + Skip, built via MiddlewareResult.Continue(item) (keep/replace) or MiddlewareResult.Drop<T>() (remove).
• WithMiddleware(...) extensions on IAsyncEnumerable<T> — a single middleware or an ordered chain (each sees the previous one's output; a drop short-circuits the rest of the chain).

var cleaned = extractor.ExtractAsync(ct)
    .WithMiddleware(new LoggingMiddleware<Order>())
    .WithMiddleware(new ValidationMiddleware<Order>());   // drops invalid orders

Because it decorates a stream, it composes anywhere — extractor/transformer output, loader input, and inside an EtlPipeline:

EtlPipeline.Create().From(src).Through(s => s.WithMiddleware(m)).To(loader);

Design notes

• Dependency-free, per the lightweight-Abstractions principle — pure interfaces + decorator extensions, no new package reference.
• MiddlewareResult<T> has full value equality (satisfies CA1815); the factory methods live on a non-generic MiddlewareResult companion (CA1000-clean).
• Error interaction (Add OnItemError hook + RecordsSkipped (dead-letter / error capture) to base classes #84): middleware runs as ordinary stream stages, so a stage's OnItemError still wraps its own worker — they compose without overlap.

Verification

• 441 unit tests pass (15 new MiddlewareTests + MiddlewareResult value-semantics: transform, drop, chain order, stop-at-drop, empty chain, token flow, EtlPipeline composition, all null guards).
• Stryker 100.00 %, 0 survivors (full project; equivalents — ConfigureAwait(false), the GetHashCode formula, and the diagnostic message string — marked with justified // Stryker disable once comments per repo convention).
• Builds clean across all 11 TFMs; PublicAPI.Unshipped updated (RS0016 completeness + RS0017 correctness both validated).

Notes for release-prep (not in this PR)

Targets the 0.20.0 line (vNext-plus-one). Version bump 0.19.0 → 0.20.0 and baseline → 0.19.0 are deferred to 0.20.0 release-prep, gated on 0.19.0 publishing.

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