Gate READY on download completion; move audio to ~/.radiantwave/audio

State machine now uses DOWNLOADING_ASSETS before READY. Downloads closing
the DownloadDone channel signals checkState() to proceed. Download failure
sets downloadFailed flag which prevents READY. Audio files moved from
~/.local/share/rhemawave/audio to ~/.radiantwave/audio/.

Regression tests in application_test.go (linux build-tagged due to
network package D-Bus init). Version 0.6.

nixos/vm.nix

@@ -82,6 +82,7 @@ in
   # ── Data directory ────────────────────────────────────────────────────────
   systemd.tmpfiles.rules = [
     "d /home/kiosk/.local/share/rhemawave 0770 kiosk kiosk -"
+    "d /home/kiosk/.radiantwave/audio 0755 kiosk kiosk -"
   ];
 
   # ── RadiantWave service (via loginShellInit, same as production) ──────────
@@ -108,6 +109,8 @@ in
       # Asset sync + start app (same as production)
       mkdir -p /home/kiosk/.local/share/rhemawave
       ${pkgs.rsync}/bin/rsync -ru ${rhemaApp}/share/rhemawave/ /home/kiosk/.local/share/rhemawave/
+      mkdir -p /home/kiosk/.radiantwave/audio
+      ${pkgs.rsync}/bin/rsync -ru ${rhemaApp}/share/rhemawave/audio/ /home/kiosk/.radiantwave/audio/
 
       cage -- ${rhemaApp}/bin/rhemawave -d -s > /tmp/kiosk.log 2>&1
     fi