node: virtual graph endpoints and the Kahn-order BFS sweep (M0b core)

Per docs/zh/plans/render-pipeline-threads.md §3.8:

- oak-node/nodes/graphendpoints.rs: the GraphInput/GraphOutput
  virtual node pair — factory-registered but hidden from every create
  menu, duplicate refused, real value() semantics (the input forwards
  its feed_in row, the output publishes its tex_in as the frame).
  The input endpoint also declares a connectable feed_in port
  (documented deviation: footage/generator sources have no connectable
  inputs, so the walk needs a feeder anchor).
- graph.rs: ensure_endpoints/endpoints/is_endpoint — idempotent,
  identified by type id, default input->output edge only while the
  output's tex_in is free; remove_node refuses endpoints.
- project.rs + serializer.rs: every project graph carries the pair;
  a legacy file without endpoints migrates on load (roundtrip and
  legacy-migration tests, re-save is idempotent).
- traverser.rs: eval_graph_bfs — the endpoint-to-endpoint Kahn
  sweep. Live set = (input's forward cone U its feeder cone) INTERSECT
  (output's backward cone); multi-input nodes dequeue at zero
  in-degree over the live subgraph; deterministic ascending-id ready
  order (Graph::edges is a BTreeSet, so insertion order is
  unrecoverable — documented); time-shifted upstreams pull through
  the shared DFS memo (walk_dfs, factored out of evaluate);
  un-orderable remainder reports a named cycle; missing endpoints /
  unreachable output are errors. Eight BFS tests cover the plan's
  acceptance bullets.
- oak-render: bfs_endpoint_sweep_renders_footage_through_position —
  real clip through a real Position node via the sweep, shifted
  pixels asserted against a reference decode.
- Endpoint names localized in all eight i18n packs; storage/structure
  tests updated for the two extra nodes.

docs/zh/plans/render-pipeline-threads.md

@@ -349,6 +349,9 @@ fallback。** 解码上传与上屏共用一层 `gpuinteop` 抽象，按后端
 M1（与 M2 可并行）；M4 依赖 M2；M5 依赖 M2（YUV→RGB pass 与互操作抽象），
 可拆成并行子项。
 
+**全程验收（用户要求）**：M5 完成、整个任务收官后，跑一轮**分支覆盖率**
+（branch coverage）测量并留档，作为管线改造整体的质量闸门。
+
 ## 5. 不变量与边界
 
 - **不动**：ticket API（oak-app/oak-cli 无感）、撤销/重做、缓存磁盘格式、