docs: update roadmap after downloader abstraction

docs/ROADMAP.md

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+Current Status
+
+Completed
+✅ Phase 1 — Project Foundation
+✅ Phase 2 — CI / Tests / Quality Gates
+
+Current Phase
+🚧 Phase 3 — Download Engine
+
+Completed in this phase
+✅ Downloader Abstraction
+
+Remaining work
+⬜ yt-dlp Adapter
+⬜ Playlist Support
+⬜ Progress Reporting
+
+---
+
+## Phase 3 — Download Engine
+
+### Downloader Abstraction
+- [x] Downloader interface
+- [x] Mock downloader
+
+### yt-dlp Adapter
+- [ ] Download single video
+- [ ] Return metadata
+- [ ] Context cancellation
+- [ ] Unit tests
+- [ ] golangci-lint passes
+
+### Playlist Support
+- [ ] Parse playlists
+- [ ] Queue items
+
+### Progress Reporting
+- [ ] Progress callbacks
+- [ ] Download statistics