[Context]: add MapParallel with chunking and bounded concurrency

[rian-be]

Summary

Add MapParallel — the recommended batch-processing primitive combining mapping (Map) with bounded concurrency (MaxConcurrency) and chunking (partition batches) to cut per-item Task/AsyncLocal/GC overhead.

Goal

Provide the recommended method for batch processing (e.g. batch import), with partition-based context management and concurrency caps.

Problem

Mapping each of 1000 items into its own Task + BeginContext is GC-heavy for small in-memory jobs. There is no single call that bounds concurrency and batches work into a few context scopes.

Scope

• MapParallel(ctx, source, parallelOptions, workItem) where parallelOptions has MaxConcurrency and ChunkSize.
• Chunking: source of N items split into M partitions (ChunkSize); one BeginContext per partition → only M tasks.
• Reuses the caller-supplied SemaphoreSlim limits across chunks (static MaxConcurrency semaphores).
• Sample: import 10,000 products in chunks of 100 with MaxConcurrency = 4.
• Unit + benchmark tests (per performance section 2.5).

Design Expectations

• Composes Map + MaxConcurrency into a single, recommended primitive.
• Chunking reduces GC/allocations for small jobs.
• Result order defaulting to ordered; opt-out reserved for the Ordered issue.
• Async-only.

Acceptance Criteria

• Bounded concurrency respected across chunks.
• Partitioning runs fewer tasks than items and still maps all items.
• Benchmark shows chunking reduces overhead vs per-item mapping.

Non-Goals

• No retry / partial-failure aggregation (Polly, TryEach).
• No graph orchestration (DAG).
• No streaming (IAsyncEnumerable).