build(ffmpeg): static GPL FFmpeg 8.0 via project script + FFMPEG_DIR

- tooling/ffmpeg/build-ffmpeg.sh builds release/8.0 static+PIC into
  .cache/ffmpeg: GPL/version3, every free-license external codec lib
  probed via pkg-config (enabled when present), per-OS hardware
  acceleration (VideoToolbox/AudioToolbox, VAAPI/VDPAU/libdrm,
  D3D11VA/DXVA2/MediaFoundation, nvenc when ffnvcodec exists)
- tooling/install-deps.sh installs those libraries on Homebrew / MSYS2
  UCRT64 / Debian-Ubuntu / Fedora / Arch; nothing in the build sudo's
- ffmpeg-next's own build feature is unusable (every crate-version to
  FFmpeg-release pairing is broken upstream: 9.0.0->FF9 AVCodec fields,
  8.1.0->FF8.1 new enum variants, 8.0.0->FF8 FF_PROFILE rename), so
  ffmpeg-next 9 + FFmpeg 8.x headers via FFMPEG_DIR it is
- new links-crate oakffmpeg-link emits the static FFmpeg's transitive
  link flags from its .pc files (cargo only propagates them from links
  crates, and rustc prunes the flags unless the rlib is referenced —
  hence the force_link statics)
- docs/build.md updated for the Rust workspace flow

tooling/install-deps.sh

@@ -0,0 +1,93 @@
+#!/usr/bin/env bash
+# Oak Video Editor - Non-Linear Video Editor
+# Copyright (C) 2026 Oak Team
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Installs the system dependencies of the Oak Rust workspace:
+# the free-license external codec/filter libraries FFmpeg is configured
+# with (see crates/oakcodec/Cargo.toml), plus the build tools. FFmpeg
+# itself is built from source by cargo (ffmpeg-next `build` feature) and
+# is NOT installed here.
+#
+# Supported: Homebrew (macOS), MSYS2 UCRT64 (Windows), Debian/Ubuntu,
+# Fedora, Arch. Run it yourself — nothing in the build invokes it
+# automatically (it needs sudo on Linux).
+#
+# Usage: tooling/install-deps.sh
+
+set -euo pipefail
+
+run() { echo "+ $*"; "$@"; }
+
+if [[ "$OSTYPE" == msys* || "$OSTYPE" == cygwin* || -n "${MSYSTEM:-}" ]]; then
+	if [ "${MSYSTEM:-}" != "UCRT64" ]; then
+		echo "Please run this from the MSYS2 UCRT64 shell (MSYSTEM=$MSYSTEM)." >&2
+		exit 1
+	fi
+	run pacman -S --needed --noconfirm \
+		mingw-w64-ucrt-x86_64-toolchain mingw-w64-ucrt-x86_64-pkgconf \
+		mingw-w64-ucrt-x86_64-nasm \
+		mingw-w64-ucrt-x86_64-x264 mingw-w64-ucrt-x86_64-x265 \
+		mingw-w64-ucrt-x86_64-dav1d mingw-w64-ucrt-x86_64-libvpx \
+		mingw-w64-ucrt-x86_64-openh264 mingw-w64-ucrt-x86_64-openjpeg2 \
+		mingw-w64-ucrt-x86_64-libtheora mingw-w64-ucrt-x86_64-libwebp \
+		mingw-w64-ucrt-x86_64-lame mingw-w64-ucrt-x86_64-opus \
+		mingw-w64-ucrt-x86_64-libvorbis mingw-w64-ucrt-x86_64-speex \
+		mingw-w64-ucrt-x86_64-snappy mingw-w64-ucrt-x86_64-libass \
+		mingw-w64-ucrt-x86_64-freetype mingw-w64-ucrt-x86_64-fribidi \
+		mingw-w64-ucrt-x86_64-fontconfig mingw-w64-ucrt-x86_64-gnutls
+	exit 0
+fi
+
+case "$(uname -s)" in
+	Darwin)
+		run brew install pkg-config nasm \
+			x264 x265 dav1d libvpx openh264 openjpeg libtheora libwebp \
+			lame opus libvorbis speex snappy libass freetype fribidi \
+			fontconfig gnutls
+		;;
+	Linux)
+		if command -v apt-get >/dev/null; then
+			run sudo apt-get update
+			run sudo apt-get install -y build-essential pkg-config nasm \
+				libx264-dev libx265-dev libdav1d-dev libvpx-dev \
+				libopenh264-dev libopenjp2-7-dev libtheora-dev libwebp-dev \
+				libmp3lame-dev libopus-dev libvorbis-dev libspeex-dev \
+				libsnappy-dev libass-dev libfreetype-dev libfribidi-dev \
+				libfontconfig-dev libgnutls28-dev
+		elif command -v dnf >/dev/null; then
+			run sudo dnf install -y gcc gcc-c++ pkgconf-pkg-config nasm \
+				x264-devel x265-devel dav1d-devel libvpx-devel \
+				openh264-devel openjpeg2-devel libtheora-devel libwebp-devel \
+				lame-devel opus-devel libvorbis-devel speex-devel \
+				snappy-devel libass-devel freetype-devel fribidi-devel \
+				fontconfig-devel gnutls-devel
+		elif command -v pacman >/dev/null; then
+			run sudo pacman -S --needed --noconfirm base-devel pkgconf nasm \
+				x264 x265 dav1d libvpx openh264 openjpeg2 libtheora libwebp \
+				lame opus libvorbis speex snappy libass freetype2 fribidi \
+				fontconfig gnutls
+		else
+			echo "Unsupported Linux distribution (need apt-get, dnf or pacman)." >&2
+			exit 1
+		fi
+		;;
+	*)
+		echo "Unsupported platform: $(uname -s)" >&2
+		exit 1
+		;;
+esac
+
+echo "Done. 'cargo build' now compiles FFmpeg 8.1 from source with these libraries."