A4 (#170): join the two halves of an unreadable join clip, and record why the catch arm stays device-only

The ticket asked for two things. One of them is not reachable from the JVM, and saying
so is most of the value here.

**probeForConcat's catch arm cannot be provoked on this runtime.** Robolectric's
MediaExtractor never throws from setDataSource -- measured across four input shapes: an
unregistered content:// authority, a missing file://, a file of garbage bytes, and an
http:// URL. All four returned normally with trackCount = 0. So a failed read arrives as
an empty track list rather than as an exception and reaches the same
ConcatInput(null, null, 0, 0, 0) by the other road. The catch stays covered only by
ConcatEngineTest on a device. The test file says this rather than implying the arm is
handled.

**What is reachable, and was genuinely missing, is the span.** Both halves were already
covered and neither reached the other: MediaProbeTrackWalkTest pins what concatInputFrom
makes of a track list, ConcatPlannerTest's `an unknown codec is not treated as a match`
pins what the planner does with a hand-built ConcatInput(video = null). The planner's
safety rests on the probe really producing that shape, and the hand-built fixture would
go on passing if it stopped.

Measured rather than claimed: mutating concatInputFrom's initial `video` to a non-null
placeholder leaves ConcatPlannerTest green and turns this red. Dropping the planner's
video null guard turns both red -- so that half was already held, and this file does not
claim credit for it.

The coupling itself is worth writing down: ConcatPlanner guards video against a null codec
and audio not at all, and that asymmetry is correct rather than an oversight --
MediaProbe.shortName returns a non-null String, so a null audioCodec means the track is
absent and two clips with no audio really do match, while a null videoCodec means absent
*or* unreadable. The audio check is safe because the video guard fires first on a clip
nothing could read. Nothing held that.

555 -> 556 JVM tests, 0 failures. No production code changed.

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

app/src/test/java/org/libremediaconverter/convert/UnreadableJoinInputTest.kt

@@ -0,0 +1,70 @@
+package org.libremediaconverter.convert
+
+import android.net.Uri
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.libremediaconverter.model.ConcatPlanner
+import org.libremediaconverter.model.ConcatStrategy
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+
+/**
+ * A clip in a join that nothing could read, from the probe all the way to the strategy.
+ *
+ * Both halves of this are covered already, and separately: `MediaProbeTrackWalkTest` pins what
+ * `concatInputFrom` makes of a track list, and `ConcatPlannerTest`'s
+ * `an unknown codec is not treated as a match` pins what the planner does with a hand-built
+ * `ConcatInput(video = null)`. **Nothing spanned the two**, and the span is the load-bearing part:
+ * the planner's safety rests on the probe really producing that shape, and the hand-built fixture
+ * would go on passing if it stopped.
+ *
+ * Measured rather than asserted: mutating `concatInputFrom`'s initial `video` to a non-null
+ * placeholder leaves `ConcatPlannerTest` green and turns this red.
+ *
+ * ## The asymmetry this protects
+ *
+ * `ConcatPlanner` guards its video check against a null codec (`ConcatStrategy.kt:51`) and its
+ * audio check not at all (`:54`). **That is correct, not an oversight.** `MediaProbe.shortName`
+ * returns a non-null `String`, so in `concatInputFrom` a null `audioCodec` means the track is
+ * *absent* — and two clips with no audio genuinely do match. A null `videoCodec` carries both
+ * meanings, absent or unreadable, which is why only that one is guarded.
+ *
+ * So the audio check is safe *because* the video guard fires first on a clip nothing could read.
+ * Nothing wrote that coupling down and nothing held it.
+ *
+ * ## What this deliberately does not cover
+ *
+ * `probeForConcat`'s `catch` arm (`MediaProbe.kt:300-302`). It is **not reachable on the JVM**:
+ * Robolectric's `MediaExtractor` never throws from `setDataSource`, measured across an
+ * unregistered `content://` authority, a missing `file://`, a file of garbage bytes and an `http://`
+ * URL — all four returned normally with `trackCount = 0`. So the failure arrives here as an empty
+ * track list rather than as an exception, which reaches the same `ConcatInput(null, null, 0, 0, 0)`
+ * by the other road. The catch stays device-only, and this file does not pretend otherwise.
+ */
+@RunWith(RobolectricTestRunner::class)
+class UnreadableJoinInputTest {
+
+    @Test
+    fun `a clip nothing could read probes as unknown, and an unknown clip is re-encoded`() {
+        val unreadable = MediaProbe.probeForConcat(RuntimeEnvironment.getApplication(), UNREADABLE)
+
+        assertNull("an unreadable clip proves nothing about its video codec", unreadable.videoCodec)
+        assertNull("nor about its audio codec", unreadable.audioCodec)
+        assertEquals("nor about its dimensions", 0, unreadable.width)
+        assertEquals(0, unreadable.height)
+        assertEquals(0, unreadable.frameRate)
+
+        assertEquals(
+            "a clip nothing could read is not evidence of a match with anything",
+            ConcatStrategy.REENCODE,
+            ConcatPlanner.plan(listOf(unreadable, unreadable)),
+        )
+    }
+
+    private companion object {
+        /** `content://` so the probe takes the SAF branch a real pick takes. Nothing answers it. */
+        val UNREADABLE: Uri = Uri.parse("content://test/vanished.mp4")
+    }
+}