feat: streaming query pipeline (IAsyncEnumerable, ExecuteStream, resilience) — closes #299

[iancooper]

Summary

Implements spec 012 — streaming query results — across 31 tasks and 9 phases (T001–T031).

• New IStreamQuery<TResult> marker and IStreamQueryHandler<TQuery,TResult> base contract with StreamQueryHandler<,> abstract base; TResult is the item type, not the enumerable
• IQueryProcessor.ExecuteStream<TResult> — async iterator entry point; span + handler/decorator lifetime are bound to enumeration so early break and exceptions never leak resources
• PipelineBuilder.BuildStream — resolves handlers from a dedicated StreamQueryHandlerRegistry, chains IStreamQueryHandlerDecorator instances ordered by Step descending, validates that legacy sync/async attributes are not used on stream handlers (and vice versa)
• Stream logging decorator (src/Paramore.Darker/Logging/) — logs start, yields items lazily, records item count + duration on completion, captures mid-stream faults
• UseResiliencePipelineStreamHandler (Polly v8) — wraps stream establishment and the first MoveNextAsync inside the named pipeline; items are yielded only after the pipeline succeeds, so retry produces no duplicate emission; faults after the first item propagate un-retried; failed-attempt enumerators are disposed before rethrowing
• DI wiring — ServiceCollectionStreamHandlerRegistry, AddHandlersFromAssemblies extended to scan IStreamQueryHandler<,>, new AddStreamHandlers(Action<IStreamQueryHandlerRegistry>) builder method, HandlerConfiguration carries the stream registry into QueryProcessor
• QueryProcessorBuilder — accepts a HandlerConfiguration with a stream registry, no additional changes required (T006/T008 already covered this)
• FakeQueryProcessor updated with a no-op ExecuteStream implementation
• netstandard2.0 support via conditional Microsoft.Bcl.AsyncInterfaces package reference
• AOT — no new trim/AOT-hostile reflection beyond the existing BuildAsync path; AOT publish verified green
• Documentation — new "Streaming Queries" section in README.md covering usage, DI registration, resilience, and all documented semantics

Documented semantics (key behavioural differences from ExecuteAsync)

Behaviour	Detail
Laziness	Framework never buffers; items produced on demand
Config errors	Deferred to first await foreach iteration (deliberate — avoids handler leak when caller never enumerates)
Re-enumeration	Re-executes the handler with a fresh pipeline each time (cold, not cached)
Resilience scope	Covers establishment + first item only; Timeout bounds start-up; Hedging unsupported
Legacy attributes	[RetryableQuery]/[FallbackPolicy] throw ConfigurationException on a stream handler; use [UseResiliencePipelineStream]
Caller context	A caller-supplied queryContext is scoped to a single enumeration

Test plan

• 201 core tests pass (net8.0 + net9.0) covering: laziness, cancellation mid-stream, exceptions mid-stream, early-break lifetime release, deferred config error, decorator ordering, mismatch rejection, re-enumeration, logging lifecycle, span events per step, resilience happy path, retry-no-duplicate, post-first-item fault propagation, enumerator disposal on retry, fallback stream substitution
• 39 DI extension tests pass covering: assembly scan registering stream handlers, explicit AddStreamHandlers, QueryProcessorBuilder with stream registry
• Release build clean (0 errors, 305 pre-existing warnings)
• AOT publish (net9.0, osx-arm64) succeeds with no new IL warnings beyond pre-existing BuildAsync ones

Closes #299

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