tooling/ffmpeg: mark completed builds; refresh the pairing comment

- touch $PREFIX/.build-complete after a successful install: the CI cache
  rejects a restored tree without it (a failed run's partial save) and
  rebuilds instead;
- the old comment claimed every ffmpeg-next/FFmpeg pairing is broken
  upstream. Verified locally instead: ffmpeg-next 9.0.0 compiles against
  both release/8.1 and release/9.0 (cargo check of a scratch crate and of
  oak-codec against the 9.0 build), so the pinned version is a project
  choice, not a compatibility workaround.

tooling/ffmpeg/build-ffmpeg.sh

@@ -21,12 +21,12 @@
 #   export FFMPEG_DIR="$(pwd)/.cache/ffmpeg"
 #
 # Why a script instead of ffmpeg-next's `build` cargo feature: the
-# feature clones release/<crate-version>, and every such pairing is
-# broken upstream (9.0.0 -> FFmpeg 9.0 headers removed AVCodec fields;
-# 8.1.0 -> FFmpeg 8.1 added enum variants; 8.0.0 -> FFmpeg 8.0 renamed
-# FF_PROFILE_* to AV_PROFILE_*). The one known-good pairing is
-# ffmpeg-next 9.0.0 against FFmpeg 8.x headers, so this script builds
-# release/8.0.
+# feature clones release/<crate-version>, tying the FFmpeg version to the
+# crate version; the project pins its own (FFMPEG_VERSION below) and
+# ships a known-good pairing. ffmpeg-next 9.0.0 compiles against both the
+# FFmpeg 8.x and 9.x headers — verified locally with this script's builds
+# of release/8.1 and release/9.0 — so the version below is a project
+# choice, not a compatibility workaround.
 #
 # Oak is GPL, so the GPL-licensed parts of FFmpeg and every free-license
 # external codec library are enabled. Hardware acceleration is enabled
@@ -202,6 +202,10 @@ echo ">> make -j$JOBS"
 make -j"$JOBS"
 make install
 
+# Marker for the CI cache: a restored tree without it is incomplete (a
+# failed run's partial save) and is discarded/ rebuilt.
+touch "$PREFIX/.build-complete"
+
 cat <<EOF
 
 Done. To build Oak against this FFmpeg: