Add parallel pipeline with Channel-based scheduling and cancellation support

[JusterZhu]

Summary

Replace the sequential file processing with a TPL Dataflow / Channel-based parallel pipeline. Add CancellationToken support throughout.

Scope

• \DiffPipeline\ class orchestrating diff tasks via \Channel\
• \IPipelineScheduler\ for pluggable scheduling strategies
• Configurable parallelism degree (default: Environment.ProcessorCount)
• CancellationToken propagation through all layers
• Progress reporting: \IAsyncEnumerable\ per-file status
• Thread-safe incremental output

Expected Improvements

• Directory diff: near-linear speedup with CPU core count (~4-8x on typical hardware)
• Cancellable long-running operations (no more stuck processes)
• Real-time progress feedback for UI integration

Dependencies

• Requires Refactor Differential: Abstract interfaces for pluggable differs and compression #258 (interface abstractions)