audio: select the pipewire backend explicitly and ban the pulse one

Full-system audit findings: without an explicit provider selection the
image installed real PulseAudio (Plasma Mobile hard-path), which claims
the card and ships a wireplumber fragment disabling hardware.audio -
zero sinks forever. Mirror the dev phone's world: pipewire backend meta
+ pipewire-pulse + echo-cancel in extra_packages, pulse meta banned via
apk conflict in fp6-device-tweaks. The two hand-made /etc audio
fragments (wireplumber no-ACP rule, AEC pair for VoLTE speakerphone)
move into soc-fairphone-fp6-audio.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

aports/device/soc-fairphone-fp6-audio/51-fp6-ucm.conf

@@ -0,0 +1,11 @@
+monitor.alsa.rules = [
+  {
+    matches = [ { device.name = "alsa_card.platform-sound" } ]
+    actions = {
+      update-props = {
+        api.alsa.use-acp = false
+        api.alsa.soft-mixer = false
+      }
+    }
+  }
+]