Stop telling people to stage the benchmark the one way it cannot be staged

RealMediaBenchmark's class KDoc said:

  Populate with:
    adb push <file>.mp4 /sdcard/Android/data/org.libremediaconverter/files/

Twelve lines below, the `samples` property KDoc -- on `get() = context.filesDir` -- says:

  Internal storage, not the external files dir. Files placed in the external dir by
  `adb push` or `adb shell cp` stay owned by the shell user, and the app then gets
  EACCES trying to read them -- which presents as an unparseable input rather than a
  permission problem.

Different directories, and the second exists specifically to explain why the first fails.
Anyone following the class KDoc stages files the benchmark cannot read, gets a skip, and
reads the skip as "not staged yet" -- the failure mode the property KDoc warns about, walked
into by the instruction in the same file.

The fix is not a corrected command. Restating the mechanism in a second place is what let
these drift, and a replacement command I have not executed would be the same defect with a
fresher date. The class KDoc now names [samples] as the single place that answers it.

Two things added that are checkable rather than remembered: the exact filenames the tests
look for, via [H264_SAMPLE] and [AV1_SAMPLE] -- the old text said `<file>.mp4`, so even the
right directory left you guessing -- and a note that the two skips every green E2E leg
reports are these.

Not claimed: that the benchmark misbehaves on CI. An earlier version of the ticket said so;
it was wrong, and measuring settled it -- both tests report SKIPPED on the gating legs, the
guards work, and "harmless in CI" is accurate. The failure that prompted the look is
Media3EngineTest, tracked as #102.

Closes #101.

app/src/androidTest/java/org/libremediaconverter/bench/RealMediaBenchmark.kt

@@ -36,8 +36,20 @@ import java.io.File
  *  1. that the hardware path is worth having a second engine for at all, and
  *  2. that x264's CRF is worth the GPL licence the app carries for it.
  *
- * Skips itself when the sample files are absent, so it is harmless in CI. Populate with:
- *   adb push <file>.mp4 /sdcard/Android/data/org.libremediaconverter/files/
+ * Skips itself when the sample files are absent, so it is harmless in CI — every green E2E
+ * leg reports two skips, and these are they.
+ *
+ * The two files it looks for, by exact name:
+ *
+ *  - [H264_SAMPLE] for [hardwareVersusSoftwareOnRealVideo]
+ *  - [AV1_SAMPLE] for [av1InputRoutesAccordingToDeviceDecodeSupport]
+ *
+ * **Where they go, and how, is on [samples] — read it before staging anything.** This used to
+ * carry an `adb push` line naming the external files dir, which [samples] then explains cannot
+ * work: a pushed file stays owned by the shell user and the app reads EACCES, surfacing as an
+ * unparseable input rather than a permission error. The instruction and its own refutation sat
+ * twelve lines apart. It is named in one place now rather than restated here, because restating
+ * it is what let the two drift.
  */
 @UnstableApi
 @RunWith(AndroidJUnit4::class)