fix(build): launch crash — @rpath/libz.1.dylib had no LC_RPATH

The static FFmpeg's external codec libs pull in -lz, which on this
toolchain resolves to a copy whose install name is @rpath/libz.1.dylib
(zlib-ng-compat); without an LC_RPATH entry all three binaries died in
dyld at startup. The app/cli/worker build scripts now emit
-Wl,-rpath,/usr/lib.

Also: FFMPEG_DIR moves into the committed .cargo/config.toml as a
workspace-relative [env] entry — ffmpeg-sys-next's build script cannot
read .env files, and without it the crate silently linked the shared
Homebrew FFmpeg while oakffmpeg-link emitted the static transitive
flags (mixed linkage). docs/build.md updated.

docs/build.md

@@ -23,10 +23,14 @@ This document describes how to build Oak Video Editor from source on Windows, Li
 > ```sh
 > tooling/install-deps.sh           # Homebrew / MSYS2 UCRT64 / Debian / Fedora / Arch
 > tooling/ffmpeg/build-ffmpeg.sh    # clones release/8.0, installs into .cache/ffmpeg
-> export FFMPEG_DIR="$(pwd)/.cache/ffmpeg"
 > cargo build
 > ```
 >
+> `FFMPEG_DIR` no longer needs exporting: the committed
+> `.cargo/config.toml` sets it relative to the workspace root (the
+> ffmpeg-sys-next build script cannot read `.env` files — this is the
+> only machine-agnostic way).
+>
 > External libraries are probed with `pkg-config` and silently skipped
 > when missing. `FFMPEG_DIR` is mandatory (the `oakffmpeg-link` build
 > script panics without it): silently binding a system FFmpeg risks

docs/zh/plans/riir/M14-direct-rlib.md

@@ -1,4 +1,4 @@
-# M14：前端绕过 facade 直链 rlib（纯 Rust ABI）
+ # M14：前端绕过 facade 直链 rlib（纯 Rust ABI）
 
 > 前置：单库化（single-lib.md）+ 模块 bridge/ffi 清除已完成；
 > liboakengine.dylib 的 C ABI 冻结，专供插件/外部消费者。