feat(discord-rpc): lazy Flatpak socket bridge keeps the integration opt-in

Discord-Flatpak writes its IPC socket to
\$XDG_RUNTIME_DIR/app/com.discordapp.Discord/discord-ipc-N rather than
the \$XDG_RUNTIME_DIR/discord-ipc-N path the discord-rich-presence crate
searches by default. The naive way to bridge that gap — a startup
wrapper that unconditionally symlinks every iteration before the binary
launches — works, but it pulls the integration off-script: every other
optional integration in QBZ (Last.fm, ListenBrainz, MusicBrainz, Plex)
is opt-in, with zero telemetry and zero filesystem reach until the user
flips the toggle. A startup wrapper would silently turn Discord RPC
into opt-out at the sandbox layer; the symlinks would exist for users
who never touched the integrations panel.

Move the bridge into the lazy connect path instead. The first time the
user actually connects — i.e. the first track plays after they enabled
the toggle — prepare_flatpak_discord_socket_links runs:

* No-op outside Flatpak (no FLATPAK_ID env var)
* No-op when \$XDG_RUNTIME_DIR/discord-ipc-N already exists (native
  Discord, or a previous lazy run in this Flatpak instance)
* No-op when the Discord-Flatpak source path doesn't exist (Discord
  not running)
* Otherwise, symlinks 0..9 from \$XDG_RUNTIME_DIR/discord-ipc-N to
  \$XDG_RUNTIME_DIR/app/com.discordapp.Discord/discord-ipc-N

Symlinks die with the Flatpak instance — \$XDG_RUNTIME_DIR is per-
instance, so there's no cross-session pollution. Users who never enable
Discord RPC keep the same sandbox surface as before.

Manifest: keep the static xdg-run/discord-ipc-0 permission for native
Discord, add xdg-run/app/com.discordapp.Discord:create so the lazy
symlink path is allowed when the user opts in. The two coexist; the
crate's default search path covers Discord-native, the symlinks cover
Discord-Flatpak.

cc

packaging/flatpak/com.blitzfc.qbz.yml

@@ -18,8 +18,19 @@ finish-args:
   # Native PipeWire access
   - --filesystem=xdg-run/pipewire-0:ro
 
-  # Discord IPC socket for Rich Presence (#336)
+  # Discord IPC socket for Rich Presence (#336).
+  # - The static path covers Discord installed natively (deb / rpm / AUR
+  #   builds), which write the socket directly under $XDG_RUNTIME_DIR.
+  # - The :create permission on the Discord-Flatpak app dir lets us read
+  #   the socket created by Discord-via-Flathub AND lay down our own
+  #   $XDG_RUNTIME_DIR/discord-ipc-{0..9} symlinks pointing at it. The
+  #   crate `discord-rich-presence` only knows the static path; the
+  #   symlinks are what bridge the two sandboxes. See discord_rpc.rs:
+  #   prepare_flatpak_discord_socket_links — called only after the user
+  #   opts in via the settings toggle, so users who never enable Discord
+  #   RPC never touch this dir.
   - --filesystem=xdg-run/discord-ipc-0
+  - --filesystem=xdg-run/app/com.discordapp.Discord:create
 
   # User music library
   - --filesystem=xdg-music