feat(app): multicam panel with live angle grid, switching, timeline enable

- New MulticamPanel: rows/cols angle grid with the current angle
  highlighted, click-to-switch, 1-9 switch-and-split and cmd-1-9
  switch-only shortcuts (focused-panel routed), deferred switch queue
  during playback.
- src/oakui/multicam.rs: clip->connected-sequence resolution, multicam
  state detection (selection then playhead fallbacks), per-angle frame
  requests rendered through the process backend into an LRU cache.
- Timeline clip context menu Multi-Cam checkable item wired to
  oaktimeline::multicam enable/disable with undo.
- Engine trait extended (real + mock); mock drives the real command
  path with synthesized angle frames.

crates/oakengine.bk/include/node/error.h

@@ -0,0 +1,49 @@
+/***
+
+  Oak Video Editor - Non-Linear Video Editor
+  Copyright (C) 2026 Oak Team
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#ifndef OAK_EDITOR_NODE_ERROR_H
+#define OAK_EDITOR_NODE_ERROR_H
+
+/**
+ * @brief Status and error codes shared by all oaknode C API families.
+ *
+ * Return-code convention (mirrors engine/include/oakengine/init.h):
+ * 0 (OAKNODE_OK) on success, a negative OAKNODE_E_* error code on
+ * failure. String getters return the required buffer size in bytes
+ * (including the terminating NUL) as a non-negative value instead.
+ */
+/**
+ * @brief Current ABI version stamped into every oaknode handle.
+ *
+ * Bump whenever a handle layout or the semantics of any exported
+ * function change incompatibly. Consumers should compare a handle's
+ * abi_version field against the value they were compiled with before
+ * dereferencing ctx.
+ */
+#define OAKNODE_ABI_VERSION 1
+
+#define OAKNODE_OK 0 /**< Success. */
+#define OAKNODE_E_INVALID (-30001) /**< NULL handle or invalid argument. */
+#define OAKNODE_E_STATE (-30002) /**< Call not valid in the current state. */
+#define OAKNODE_E_FAILED (-30003) /**< The underlying operation failed. */
+#define OAKNODE_E_NOT_FOUND (-30004) /**< Index out of range / entry not found. */
+#define OAKNODE_E_NOMEM (-30005) /**< Allocation failed. */
+
+#endif //OAK_EDITOR_NODE_ERROR_H