[Context]: add execution strategies (Sequential/Parallel/DedicatedThread)

[rian-be]

Summary

Add an execution Strategy pattern (SequentialStrategy, ParallelTaskStrategy, DedicatedThreadStrategy) so how a batch is processed can change without touching business code.

Goal

Let processing mode be swapped at runtime (e.g. sequential for debugging, parallel for prod) via a pluggable strategy contract.

Problem

Execution mode is hard-coded in each call. Changing between sequential (debuggable) and parallel requires rewriting call sites.

Scope

• IExecutionStrategy contract.
• SequentialStrategy: run tasks in order.
• ParallelTaskStrategy: standard Task.WhenAll.
• DedicatedThreadStrategy: for CPU-heavy work (explicit scheduling).
• Strategy selected via ExecutionOptions or registration.
• Unit tests: switching strategy changes none of the application code.

Design Expectations

• Thin; resilient; the strategy orchestrates the base primitives.
• Business delegates stay identical regardless of strategy.
• Async-only across strategies.

Acceptance Criteria

• Swappable strategy without call-site changes.
• Sequential yields deterministic order; parallel yields completion-speed results.
• Applies uniformly to batch/map under use-case.

Non-Goals

• No Parallel.ForEach blocking mode.
• No new scheduler/threadpool implementation.