Add parallel DiffPipeline with progress reporting and cancellation (#260)

[JusterZhu]

Closes #260

Summary

New DiffPipeline class for parallel file processing with progress reporting and CancellationToken support throughout.

Changes

New Files

• Pipeline/DiffPipeline.cs — parallel orchestrator using SemaphoreSlim-throttled Task.Run
• Pipeline/DiffPipelineOptions.cs — configurable parallelism, error strategy
• Models/DiffProgress.cs — progress struct with percentage, current file, error info

Design

• Throttled parallelism: SemaphoreSlim(MaxDegreeOfParallelism) limits concurrency (default: CPU cores)
• Per-file error isolation: one file failing does not stop others; errors reported via progress
• Full cancellation: CancellationToken propagated to every file-level diff
• Progress: IProgress reports completed/total, current file, percentage
• Zero new dependencies: SemaphoreSlim, IProgress<>, Task.WhenAll — all BCL built-in

Usage Example

var pipeline = new DiffPipeline(new DiffPipelineOptions { MaxDegreeOfParallelism = 8 });
var progress = new Progress<DiffProgress>(p => Console.WriteLine(p));
await pipeline.CleanAsync(oldApp, newApp, patchDir, progress, ct);