feat(engine): clip move, clip effect_input, mandatory static FFmpeg

- oakengine_sequence_move_clip implemented for real (oaktimeline
  TrackMoveBlockCommand; fixes the graph-ownership/gap-anchor/ripple
  trim bugs the stub was hiding); same-track via the frozen C ABI,
  cross-track supported by the module command
- oaknode clip blocks now declare a tex_in texture input and set
  effect_input to it, so timeline clips can host effect chains; facade
  test covers effect insert/remove on a real clip
- oakffmpeg-link: FFMPEG_DIR is now mandatory with a clear panic (a
  Homebrew upgrade left the system ffmpeg .pc pointing at a deleted
  dav1d Cellar path, breaking links); reads a git-ignored workspace
  .env for IDEs that cannot inject env vars (RustRover); links the C++
  stdlib for C++ codec libs (svt-av1)
- oakengine re-exports oaknode so tests share one crate instance;
  it_node uses the direct instance's value type where it calls the
  module FFI (the --workspace dev-dependency feature split builds
  oaknode twice)

docs/build.md

@@ -25,8 +25,12 @@ This document describes how to build Oak Video Editor from source on Windows, Li
 > ```
 >
 > External libraries are probed with `pkg-config` and silently skipped
-> when missing. Unset `FFMPEG_DIR` to fall back to the system
-> `pkg-config` FFmpeg (must be 8.x for ffmpeg-next to compile).
+> when missing. `FFMPEG_DIR` is mandatory (the `oakffmpeg-link` build
+> script panics without it): silently binding a system FFmpeg risks
+> stale `.pc` paths after package upgrades. IDEs that cannot inject
+> environment variables into cargo (RustRover) can instead put
+> `FFMPEG_DIR=...` (and `PKG_CONFIG_PATH=...` where needed) into a
+> `.env` file at the workspace root — it is git-ignored.
 > ffmpeg-next's `build` cargo feature is deliberately not used: it
 > clones release/<crate-version>, and every such pairing is broken
 > upstream (9.0.0 → FFmpeg 9.0 removed AVCodec fields; 8.1.0 → FFmpeg