Read the progress percentage FFmpeg has always been computing (#229)

FFmpegEngine derives progress as stats.time / durationMs * 100, and the
statistics callback runs on every conversion in FFmpegEngineTest -- they
all pass durationMs = 3_000. But every call site omits onProgress, so
nothing on any source set had ever looked at the number. Replacing percent
with a constant reddened nothing.

What already existed covers the plumbing downstream and not this: #196
covered the worker's progress lambda with a fake engine that reports
whatever the test tells it to, and ProgressNotificationTest covers the
throttling the same way. The arithmetic was the one part with no reader.

The new test passes 30 s as the duration for a fixture that is exactly
3.000 s, so the conversion still encodes the whole clip and the reported
percentage tops out around 10 rather than 100.

That is what makes it bite. A range check alone is worthless: a constant 0
satisfies both "every value is in 0..100" and "the values never go
backwards", and so does a list of [0, 100]. Pinning the band rejects every
constant, and because the band sits a tenth of the way up it also rejects
an implementation that ignores durationMs, which would report ~100 for the
same run. The bound is loose -- 5..25 for an expected 10 -- because the
last statistics callback can land slightly before the final frame.

Verified on a local API 34 emulator: 61 tests, 0 failures, 3 skipped.

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

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

@@ -139,6 +139,58 @@ class FFmpegEngineTest {
         assertEquals("OggS", magic)
     }
 
+    /**
+     * The percentage itself, which every other test in this class computes and none of them reads.
+     *
+     * `FFmpegEngine` derives progress as `stats.time / durationMs * 100`, and the statistics
+     * callback runs on every conversion here — but every call site omits `onProgress`, so until
+     * this test nothing on any source set had ever looked at the number (#229). #196 covered the
+     * *worker's* progress lambda, and did it with a fake engine that reports whatever the test
+     * tells it to; `ProgressNotificationTest` covers throttling the same way. The arithmetic was
+     * the one part with no reader.
+     *
+     * ## Why the duration is deliberately wrong
+     *
+     * `sample_h264.mp4` is exactly 3.000 s, and this passes **30 s** as the duration. So the
+     * conversion still encodes the whole clip, `stats.time` still climbs to about 3000 ms, and the
+     * reported percentage tops out around **10** rather than 100.
+     *
+     * That is what makes the assertion bite. A range check alone is worthless here: replacing
+     * `percent` with a constant `0` satisfies "every value is in 0..100" and "the values never go
+     * backwards", and so does a list of `[0, 100]`. Pinning the *band* rejects every constant, and
+     * — because the band is a tenth of the way up — it also rejects an implementation that ignores
+     * `durationMs`, which would report ~100 for the same run.
+     *
+     * The bound is deliberately loose (5..25 for an expected 10). The last statistics callback can
+     * land slightly before the final frame, so the peak is "about 3000 ms of a claimed 30 000",
+     * not exactly it.
+     */
+    @Test
+    fun progressIsReportedAsAFractionOfTheDurationItWasGiven() {
+        val seen = mutableListOf<Int>()
+        val out = outputFor("out_progress.mp4")
+        runBlocking {
+            engine.run(
+                request = ConversionRequest(spec = OutputFormat.MP4_H264.spec, quality = QualityTier.BEST),
+                inputPath = input.absolutePath,
+                output = out,
+                // Ten times the fixture's real 3 s. See the KDoc.
+                durationMs = 30_000,
+                onProgress = { percent -> seen += percent },
+            )
+        }
+
+        assertTrue("the statistics callback never reported progress", seen.isNotEmpty())
+        assertTrue("progress out of range: $seen", seen.all { it in 0..100 })
+        assertEquals("progress went backwards: $seen", seen.sorted(), seen)
+        // The band. Rejects any constant, and rejects ignoring durationMs (which would read ~100).
+        val peak = seen.max()
+        assertTrue(
+            "3 s of media against a claimed 30 s should peak near 10%, got $peak from $seen",
+            peak in 5..25,
+        )
+    }
+
     // --- the quality tier the GPL licence was taken for --------------------
 
     @Test