Simplify download pipeline architecture

[ZenonEl]

Description

Reduce download pipeline from 4 abstraction layers to 2. Remove dead code, improve error classification.

Tasks

• Delete CliDownloadExecutor, ArgumentBuilder, ProcessRunner, RetryOrchestrator
• Create DownloadService — single orchestrator for downloader selection, retry, proxy
• Create DownloadErrorClassifier with regex patterns for 403, 429, disk full, auth errors, geo-block
• Refactor PostProcessors/ (FfmpegTranscoder, VideoSplitter) to use Result<T>
• Refactor ProxyPolicyManager and RetryPolicyManager to use Result<T>
• Update downloader-config.yaml.example

Phase

Phase 2.3-2.4 — Download Pipeline

[ZenonEl]

Already resolved by previous PRs (#15, #23). Old pipeline abstractions (CliDownloadExecutor, RetryOrchestrator, ProcessRunner, ArgumentBuilder) were removed in cleanup PR. YoutubeDLSharp replaced the manual process spawning.