Updater: per-platform manifest assets, Windows apply tested on NTFS

The manifest gains `assets` (linux-x64 / windows-x64 → archive URL);
top-level `url` stays the Linux tarball for 0.2.1 installs. A Windows-
only test applies a bsdtar zip over a running exe (PING.EXE copy) and
checks it is parked as .old and swept next start; Linux-layout tests
(exec bits, symlinks) skip on Windows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/pubspec.lock

@@ -723,14 +723,14 @@ packages:
       path: "../../packages/core"
       relative: true
     source: path
-    version: "0.1.0"
+    version: "0.2.1"
   sync_relay_server:
     dependency: "direct main"
     description:
       path: "../../packages/server"
       relative: true
     source: path
-    version: "0.1.0"
+    version: "0.2.1"
   synchronized:
     dependency: transitive
     description: