docs: current bilingual build guide; retire the C++/CMake one

docs/build.md + docs/zh/build.md rewritten for the Rust workspace:
project-built FFmpeg 8.1 (.cargo/config.toml presets FFMPEG_DIR),
vendored static OCIO on Linux/macOS vs MSYS2 dynamic OCIO on Windows
(with the OCIO_INSTALL_DIR/OCIO_RS_LINK env), the Windows GNU toolchain
requirements (MSYS2 Rust, RUSTFLAGS=-C link-args=-lmsvcrt for the
mingw-w64 _assert forwarding, unset INCLUDE/LIB), Linux audio dev
packages and xvfb headless testing, container packaging, and a
troubleshooting section. The macOS-only guides gain a deprecation
pointer. Also correct two stale comments in tooling/install-deps.sh
(FFmpeg is built by tooling/ffmpeg/build-ffmpeg.sh, not by cargo).

docs/build_macos.md

@@ -1,5 +1,9 @@
 # macOS Build Guide
 
+> **Deprecated (2026):** this guide describes the old C++/CMake build.
+> The current Rust workspace build is documented in
+> [`build.md`](build.md) (中文：[`zh/build.md`](zh/build.md)).
+
 This document describes how to build Oak Video Editor from source on macOS.
 
 For the Chinese version, see [`build-macos-zh.md`](zh/build_macos-zh.md).