Merge Media3's MP4-only correction into the remux branch

CI on the parent branch proved that four of the five containers the router
claimed for Media3 cannot be written by Transformer at all: WebmMuxer, OggMuxer,
WavMuxer and AacMuxer each throw UnsupportedOperationException from
addMetadataEntry, which MuxerWrapper calls for every metadata entry on the track
format.

Consequences here beyond the merge itself:

- MEDIA3_MUXABLE_VIDEO and MEDIA3_MUXABLE_AUDIO drop to a single MP4 entry.
  Every other container is already on its way to FFmpeg before those maps are
  consulted.
- Reason.WEBM_CODEC_UNSUPPORTED is removed. WebM now fails the container check
  first, so nothing could ever produce that reason, and a routing reason no code
  path can reach is worse than no reason at all.
- Media3Muxers gains null branches for the six containers this branch adds. MOV
  is among them despite being MP4's own family: Mp4Muxer exposes no QuickTime
  file format.
- The README no longer claims Media3 writes five containers.

The remux behaviour this branch exists for is unaffected: MKV -> MP4 was always
the hardware direction, because Media3 reads Matroska but has never been able to
write it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/androidTest/java/org/libremediaconverter/convert/Media3EngineTest.kt

@@ -109,48 +109,6 @@ class Media3EngineTest {
         }
     }
 
-    /**
-     * WAV output, which reaches Media3 for the same reason M4A does.
-     *
-     * `Container.WAV` is in the router's Media3 set, but until the engine passed a muxer factory
-     * the only muxer Transformer ever used was the MP4 one — so asking for WAV produced an MP4.
-     * Asserting the RIFF header proves the container, not merely that a file appeared; this
-     * follows what `FFmpegEngineTest` already does for its formats.
-     */
-    @Test
-    fun wavExportWritesARiffHeader(): Unit = runBlocking {
-        val wav = File(context.cacheDir, "out_audio.wav")
-        wav.delete()
-        try {
-            engine.transcode(Uri.fromFile(input), wav, ConversionRequest(OutputFormat.WAV.spec))
-
-            assertTrue("export produced no file", wav.exists() && wav.length() > 0)
-            assertEquals("RIFF", String(wav.readBytes().copyOfRange(0, 4), Charsets.US_ASCII))
-        } finally {
-            wav.delete()
-        }
-    }
-
-    /**
-     * Ogg/Opus output, the third container the router claims for Media3.
-     *
-     * Asserted by magic bytes rather than `MediaExtractor`: platform extractor support for raw
-     * Ogg is inconsistent across the API levels in the CI matrix, and "OggS" is unambiguous.
-     */
-    @Test
-    fun opusExportWritesAnOggHeader(): Unit = runBlocking {
-        val ogg = File(context.cacheDir, "out_audio.opus")
-        ogg.delete()
-        try {
-            engine.transcode(Uri.fromFile(input), ogg, ConversionRequest(OutputFormat.OPUS.spec))
-
-            assertTrue("export produced no file", ogg.exists() && ogg.length() > 0)
-            assertEquals("OggS", String(ogg.readBytes().copyOfRange(0, 4), Charsets.US_ASCII))
-        } finally {
-            ogg.delete()
-        }
-    }
-
     /**
      * Regression guard for the Transformer threading trap.
      *