Add TODO: add-on WADs and folder links behind redirects

TODO.md

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+# TODO
+
+## Add-ons (PWADs)
+
+The launcher only starts the main games; add-on WADs found in archives are skipped. Order of work:
+
+- [ ] Pick an add-on in the launcher and load it on top of its game (`-file`), import PWADs from
+      archives and links, and give each add-on its own saves.
+- [ ] *No Rest for the Living* (`nerve.wad`) and the *Master Levels* (`masterlevels.wad`) from the
+      2024 re-release: classic-format DOOM II maps, so they should run once loading works. Check
+      their level order and secret exits.
+- [ ] *SIGIL* and *SIGIL II* (`sigil.wad`, `sigil2.wad`): the re-release versions are episodes 5 and
+      6, which need episode, intermission and sky support beyond episode 4.
+- [ ] *Legacy of Rust* (`id1.wad` with `id1-*.wad` and `id24res.wad`): needs the id24 extensions
+      (new weapons, monsters and the rest of the spec). Largest of these.
+
+## Importer
+
+- [ ] Folder links behind a redirect (for example a shorturl.at link to a folder): the listing's
+      relative links are resolved against the typed address, not the address the redirect lands
+      on, so the files cannot be opened. Learn the final address (`sceHttpSetRedirectCallback`,
+      `CURLINFO_EFFECTIVE_URL` on the PC) and resolve against it. Short links straight to a file
+      already work.