feat: Download System v2 — complete rewrite of download pipeline
Replace monolithic download-manager.ts (9500 lines) with 7 focused modules:
- error-classifier.ts: 25+ typed DownloadErrorKind enum, classifier functions
for network/HTTP/debrid/extraction errors — no more string matching
- retry-manager.ts: Declarative per-error-kind retry policies, exponential
backoff, shelving after 15 failures, state export/import
- stream-writer.ts: HTTP stream → file with pre-resume validation, stall
detection, NTFS-aligned buffered writing, Range-ignored detection
- pipeline.ts: Single download lifecycle (unrestrict → stream → verify),
throws typed errors, caller decides retry strategy
- post-processor.ts: Extraction state machine with hard caps (3 attempts
per archive, 5 rounds per package), no infinite loops
- scheduler.ts: Queue management with priority-based slot allocation,
heartbeat stall detection, global watchdog, provider cooldowns
- download-manager.ts: Drop-in orchestrator (~1500 lines), same public API
Fixes:
1. Hanging downloads: heartbeat-based stall detection + global watchdog
2. Wrong error classification: typed enum at point of origin
3. Unreliable resume: file size vs tracker validation, Range-ignored detection
4. Extraction loops: bounded retries with state machine
215 new unit tests for error-classifier and retry-manager (all passing).
Build compiles cleanly. Same IPC interface — UI unchanged.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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