Record native Linux CPU media helper validation

docs/VALIDATION.md

@@ -5,7 +5,18 @@ inference, FFmpeg decoding and WebView tests use CPU only. UI launches set
 `WEBVIEW2_ADDITIONAL_BROWSER_ARGUMENTS='--disable-gpu --disable-accelerated-video-decode'`
 and `SEARCHI_FORCE_CPU=1`.
 
-## Current integration checkpoint (2026-10-12)
+## Native Linux media checkpoint (2026-10-13)
+
+Pinned BtbN Linux x86-64 FFmpeg/FFprobe archive size and SHA-256 verification
+passed under Ubuntu 24.04 WSL. Both native tools report the pinned version;
+FFprobe inspected the four-second speech fixture and FFmpeg decoded it to a
+null sink using one CPU thread with hardware acceleration disabled. Exact
+tool/fixture hashes and scope are recorded in
+[Linux media evidence](evidence/linux-media-runtime.json). This verifies decoder
+startup and WAV decoding only; playback, other codecs, desktop packaging,
+distribution ABI coverage and GPU operation remain release gates.
+
+## Integration checkpoint (2026-10-12)
 
 The subsequent rebuild-policy fix passes the Windows workspace suite (93
 passed, 12 explicitly ignored) and strict service Clippy. A separately executed