engine: sequence parameters dialog migrates to the facade

- new facade API: video params ex (dimensions/rate/par/interlacing/
  preview format/divider), audio params, preview divider, and
  undoable-flagged setters mirroring the dialog's dual undo/no-undo
  modes; label setting gains node_set_label_ex
- the dialog's own SequenceParamCommand is gone; accept now issues
  facade calls (per-field commands, unchanged fields skipped)
- preset system stays UI-side by design: its flat XML schema never
  touches engine objects
- the auto-cache checkbox maps to the engine's existing stub (no undo
  noise)

engine/include/oakengine/node.h

@@ -140,6 +140,17 @@ OAKENGINE_API int oakengine_node_get_label(const OakEngineNode *self,
 OAKENGINE_API int oakengine_node_set_label(OakEngineNode *self,
 										   const char *label);
 
+/**
+ * @brief Set the node's user label with an explicit undoable flag.
+ *
+ * `undoable` != 0 behaves like oakengine_node_set_label() (one undoable
+ * NodeRenameCommand on the global undo stack, degrading to a direct
+ * rename when the engine is not initialized); 0 renames directly with no
+ * undo entry.
+ */
+OAKENGINE_API int oakengine_node_set_label_ex(OakEngineNode *self,
+											  const char *label, int undoable);
+
 /* ---- Input introspection ---------------------------------------------------- */
 
 /**