Check that FLAC and Opus are FLAC and Opus (#228)

encodesFlacLosslessAudio and encodesOpus asserted only that a non-empty
file appeared. Five siblings in the same class check what is in it --
encodesWav reads RIFF two lines away, and GIF, Matroska, MP3, H.264 and
H.265 all assert a container marker or a track MIME.

convert() throws on a non-zero return code, so these two did prove the
command ran. What they could not distinguish is the command running and
producing the wrong thing, which is a failure mode this codebase has
already had once: F1 in docs/coverage-read-findings.md records a live
Vorbis arm in FFmpegCommandBuilder that ContainerCapabilities says cannot
exist. Pointing OutputFormat.FLAC's arm at pcm_s16le left both tests green.

Four bytes each, in the idiom the class already uses. OutputFormat.FLAC is
Container.FLAC, whose ffmpeg format is "flac", so the file opens with the
native stream marker fLaC. OutputFormat.OPUS is Container.OGG -- "ogg" --
so it is an Ogg stream and opens with OggS. Verified against both muxers
directly rather than assumed from the codec name; the marker belongs to the
container, so it does not vary with the ffmpeg build.

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

app/src/androidTest/java/org/libremediaconverter/ffmpeg/FFmpegEngineTest.kt

@@ -113,6 +113,11 @@ class FFmpegEngineTest {
     fun encodesFlacLosslessAudio() {
         val out = convert(OutputFormat.FLAC)
         assertTrue("no FLAC produced", out.exists() && out.length() > 0)
+        // "fLaC", the native FLAC stream marker. Without this the test passed on any non-empty
+        // file, so a builder arm emitting the wrong encoder into a .flac name shipped green
+        // (#228) -- the same shape the five assertions above already guard against.
+        val magic = out.inputStream().use { String(it.readNBytes(4), Charsets.US_ASCII) }
+        assertEquals("fLaC", magic)
     }
 
     @Test
@@ -127,6 +132,11 @@ class FFmpegEngineTest {
     fun encodesOpus() {
         val out = convert(OutputFormat.OPUS)
         assertTrue("no Opus produced", out.exists() && out.length() > 0)
+        // OutputFormat.OPUS is Container.OGG, so the file is an Ogg stream: "OggS" (#228).
+        // Deliberately the container marker rather than the codec -- it is what the other
+        // container-level assertions in this class check, and it is four bytes at offset 0.
+        val magic = out.inputStream().use { String(it.readNBytes(4), Charsets.US_ASCII) }
+        assertEquals("OggS", magic)
     }
 
     // --- the quality tier the GPL licence was taken for --------------------