Add buzz 0.5.28: Block's desktop workspace for humans and agents (#919)

Buzz (https://github.com/block/buzz, Apache-2.0) is a Tauri desktop app
on a Nostr relay. Upstream ships Linux builds for x86_64 only, so this
builds the desktop-v0.5.28 tag from source for x86_64 and aarch64,
following upstream's release-linux job: the six sidecar binaries from
the root workspace, then the app with --features mesh-llm. The updater
stays compiled out (its key and endpoint are unset), so pacman owns
updates. On the fast ring; the watch follows desktop-v* releases.

On aarch64, -march gains +crypto so sha2-asm can assemble its SHA
instructions; sha2 still detects them at runtime.

Built natively for aarch64 with bin/build and launched with isolated
HOME/XDG dirs: the window opens and renders the bundled frontend.

pkgbuilds/buzz/buzz.desktop

@@ -0,0 +1,10 @@
+[Desktop Entry]
+Type=Application
+Name=Buzz
+Comment=Workspace where humans and AI agents build together
+Exec=buzz-desktop %u
+Icon=buzz-desktop
+Terminal=false
+StartupWMClass=buzz-desktop
+Categories=Network;InstantMessaging;Chat;
+MimeType=x-scheme-handler/buzz;