Add perplexity, the official Perplexity desktop app

Tracks Perplexity's own Debian repository, the feed the app updates
itself from, via .omarchy/upstream.sh -- same shape as
openai-codex-desktop. pkgver carries the build number from the pool
filename because the index's Version field drops it and upstream
rebuilds under the same marketing version; the pool wants the '+'
percent-encoded. The launcher replaces the postinst symlink pacman
never creates and defaults Chromium to Wayland.

pkgbuilds/perplexity/.omarchy/package.json

@@ -0,0 +1,4 @@
+{
+  "source": "local",
+  "release_ring": "fast"
+}