Add docs/tartube-spec.md: structured analysis of Tartube's codebase

To make the "surpass Tartube" roadmap concrete, document Tartube's
v2.5.231 codebase as a specification we can refer to:

- Module map: 146k lines of Python across 19 modules, with per-module
  responsibilities (mainapp / mainwin / config / downloads / media /
  options / wizwin / process / refresh / tidy / info / updates /
  ffmpeg_tartube / formats / ttutils / dialogue / files / xdg).
- Data model: container hierarchy (Folder → Channel | Playlist → Video)
  with N-level Folder nesting, system folders that act as smart filters
  (Bookmarks, Favourites, Waiting, New, Live, Missing, Recent,
  Temporary), and the full per-video flag inventory.
- OptionsManager: the headline feature — 164 distinct yt-dlp flags
  attachable to any container or video with cascading resolution. The
  spec lists the option groups (Behaviour, Network, Playlist, Filters,
  Download, Filesystem, Auth, Anti-bot, Format, Subtitles,
  Post-processing, Cookies, Output, Tartube-specific) and points to a
  concrete SQLite + Rust struct design for our Phase 1.1.
- Seven threaded operation managers (Download, Update, Refresh, Tidy,
  Info, Process, Livestream/Stream) and three downloader strategies
  (VideoDownloader, ClipDownloader, StreamDownloader).
- Custom downloads (named recipes) and profiles (saved container
  groups) — feeding our Phase 1 + 3.
- UI surface inventory: five primary tabs (Videos, Progress, Classic
  Mode, Drag and Drop, Errors), ~80 specialised dialogs, three
  wizards (Setup, Import YT subs, Tutorial), and the ~30-tab
  SystemPrefWin.
- SponsorBlock + chapter persistence (slice_list / stamp_list) — a
  Phase 1 add-on after per-channel options ship.
- Notable corners: master/slave dbid dedup, RSS-driven discovery,
  external_dir per-channel storage, livestream message parsing, full
  gettext i18n.
- Things Tartube does better (15 items, mapped to roadmap entries),
  things we already do better (10 items, to keep momentum honest).
- A 12-checkbox "Tartube parity" milestone definition — when all are
  ticked, the comparison table at the top of ROADMAP.md goes from
  8-ahead-8-behind to 8-ahead-0-behind, and "surpass" begins.

ROADMAP.md gains a pointer to the spec just under the north-star line.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ROADMAP.md

@@ -4,6 +4,11 @@
 
 **Surpass [Tartube](https://github.com/axcore/tartube) in every dimension.**
 
+A structured analysis of Tartube's codebase, data model, operations, and
+configuration surface lives at [`docs/tartube-spec.md`](docs/tartube-spec.md).
+It enumerates the exact features we need to match — every Phase 1 item
+below traces back to a specific Tartube subsystem.
+
 Tartube is the mature open-source yt-dlp GUI and the obvious benchmark for a
 project in this space. yt-offline has architectural advantages Tartube can't
 catch up on quickly (Rust + axum + a real web UI + bundled toolchain + a