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.

crates/oak-node/src/serializer.rs

@@ -662,6 +662,12 @@ fn load_project_body(
 	// reattach each child to its bin folder.
 	resolve_folder_children(&mut project.graph, &id_map);
 
+	// Migration for projects saved before the endpoints existed (or whose
+	// file omitted them): create whatever is missing and default-wire the
+	// pair. A file that already carries both endpoints — and possibly a
+	// hand-wired output — is left untouched.
+	project.graph.ensure_endpoints();
+
 	Ok(id_map)
 }