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/50-echo-cancel.conf

@@ -0,0 +1,30 @@
+# Acoustic echo cancellation for VoLTE speakerphone calls (journal/ims.md s43).
+# Creates a virtual pair: ec_sink (play here; forwarded to the speakers and
+# used as the AEC reference) and ec_source (mic with the speaker signal
+# subtracted, WebRTC AEC). capture/playback are pinned to the hardware nodes
+# so making ec_* the defaults cannot loop the module into itself.
+context.modules = [
+    {   name = libpipewire-module-echo-cancel
+        args = {
+            library.name = aec/libspa-aec-webrtc
+            source.props = {
+                node.name = "ec_source"
+                node.description = "Echo-cancelled microphone"
+            }
+            sink.props = {
+                node.name = "ec_sink"
+                node.description = "Echo-cancelled speakers"
+            }
+            capture.props = {
+                node.name = "ec_capture"
+                target.object = "alsa_input.platform-sound.capture.1.0"
+                node.passive = true
+            }
+            playback.props = {
+                node.name = "ec_playback"
+                target.object = "alsa_output.platform-sound.playback.0.0"
+                node.passive = true
+            }
+        }
+    }
+]