Store adapters

internal/service/store.DefaultRegistry registers nine protocols: sparkle, electron, tauri, squirrel, clickonce, appimage, winget, msix, fdroid.

HTTP path /store/{protocol}/{listing_slug} (optional /{doc}). Missing listing is plaintext 404. Listing identity is the slug. line_full uses store_full, not hash-root full. Feeds must call the same SelectTarget as native check (anonymous, no device_id).