Record native Linux CPU media helper validation

tools/media-runtime/README.md

@@ -16,7 +16,12 @@ resources directory. Executables and archives are ignored by Git. No global
 installation occurs. Movie/audio decoding runs in bounded subprocesses.
 
 Windows x86-64/ARM64 and Linux x86-64/ARM64 have pinned artifacts. Only Windows
-x86-64 has been executed here. A vetted macOS decoder build/staging path remains
+x86-64 and Linux x86-64 have executed CPU checks. Linux checks verify pinned
+archive provenance, both tool versions, a four-second speech WAV probe and
+decoding to a null sink with hardware acceleration disabled. This does not
+certify playback or a codec matrix; see
+[Linux evidence](../../docs/evidence/linux-media-runtime.json).
+A vetted macOS decoder build/staging path remains
 a release gate; generic media code can accept native tools through explicit paths.
 The selected build is FFmpeg 9.0.2 with LGPL configuration. Keep original notices
 and corresponding source/build provenance with distribution; installer licensing