fix(player): pin loudnorm output to 48 kHz float to stop startup audio stutter

Audio Normalization on the mpv path made every video start with stuttering
audio on Linux: dynamic-mode loudnorm always outputs float64 at 192 kHz, so
the AO opened at f64/192k and PipeWire had to convert/resample on the
deadline-critical path (~4x the per-cycle DSP work) while playback startup
load was still settling. Appending mpv's native format filter pins the
chain back to 48 kHz float, so the conversion happens once on the buffered
decode side and the AO opens a normal stream. Integrated loudness output is
unchanged (-14 LUFS, verified via ebur128). mpv's own format filter is used
instead of lavfi aformat so the fix does not depend on which lavfi filters
each platform's bundled ffmpeg compiles in.

Addresses the normalization half of #1720; the residual startup
micro-stutter with normalization disabled is a separate regression.

test/mpv/player_native_bridge_test.dart

@@ -1003,9 +1003,9 @@ void main() {
           await expectLater(player.setAudioPassthrough(true), throwsA(isA<PlatformException>()));
 
           expect(propertyWrites.where((write) => write.$1 == 'af').map((write) => write.$2), [
-            'loudnorm=I=-14:TP=-3:LRA=4',
+            'loudnorm=I=-14:TP=-3:LRA=4,format=srate=48000:format=floatp',
             '',
-            'loudnorm=I=-14:TP=-3:LRA=4',
+            'loudnorm=I=-14:TP=-3:LRA=4,format=srate=48000:format=floatp',
           ]);
           expect(player.audioPassthroughActive, isFalse);
         } finally {