refactor(rules): split C# guidance by topic

shared/rules/csharp/references/async-and-concurrency.md

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+# C# Async and Concurrency
+
+Apply these rules when adding or changing asynchronous APIs, background work, or concurrent processing.
+
+* Use asynchronous APIs for genuinely asynchronous I/O; do not wrap synchronous work in unnecessary tasks.
+* Keep asynchronous operations async through the call chain. Avoid `.Result`, `.Wait()`, and other synchronous waits that can block threads or deadlock.
+* Accept and propagate `CancellationToken` through work that can take meaningful time or be cancelled by its caller.
+* Bound concurrency for large or unbounded workloads. Avoid creating a task per item or using uncontrolled `Task.WhenAll`; use an appropriate queue or concurrency limiter when work must be throttled.
+* Use `ValueTask` only when profiling supports it and its usage contract is clear; otherwise prefer `Task`.
+* Keep shared state safe under concurrent access and preserve cancellation and exception behavior when coordinating tasks. Prefer immutable messages or synchronization primitives with clear ownership over unsynchronized shared mutation.
+* Avoid holding locks across `await`, blocking thread-pool threads, and starting background work whose lifetime is detached from its owner.
+* For producer/consumer work, apply backpressure and bound buffered items so a fast producer cannot exhaust memory.
+* Retry only transient failures, with bounded attempts and delay. Preserve idempotency and cancellation, and avoid multiplying retries across nested layers.