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/common/handle.h

@@ -0,0 +1,69 @@
+/***
+
+  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_HANDLE_H
+#define OAK_EDITOR_HANDLE_H
+
+#include <stdint.h>
+
+/**
+ * @brief Current ABI version stamped into every oakcommon handle.
+ *
+ * Bump whenever the 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 OAKCOMMON_ABI_VERSION 1
+
+/**
+ * @brief Neutral handle convention shared by all oakcommon wrappers.
+ *
+ * Every wrapper type is a by-value struct with the same four fields:
+ *
+ *     typedef struct OakXxx {
+ *         void *ctx;                       // opaque, points to the impl
+ *         void (*addref)(void *ctx);       // atomic +1, owner-DLL code
+ *         void (*release)(void *ctx);      // atomic -1, destroys at 0
+ *         uint32_t abi_version;            // OAKCOMMON_ABI_VERSION
+ *     } OakXxx;
+ *
+ * Rules:
+ * - oakcommon_<name>_init*() returns a handle whose underlying object
+ *   has reference count 1.
+ * - Copying the struct copies the pointer, not the count: call
+ *   handle.addref(handle.ctx) for every additional long-lived copy and
+ *   handle.release(handle.ctx) (or the oakcommon_<name>_free()
+ *   convenience wrapper) when done with each copy.
+ * - release() decrements the atomic count and destroys the underlying
+ *   object when it reaches zero; the destructor runs in the DLL that
+ *   created the object, so cross-DLL handing is safe.
+ * - The struct itself carries no ownership: it is never heap-allocated
+ *   by the API, so it needs no destruction of its own.
+ * - Functions that only read a handle take it BY VALUE (OakXxx self);
+ *   an empty handle (ctx == NULL) is reported as OAKCOMMON_E_INVALID.
+ *   oakcommon_<name>_free() deliberately stays a pointer API
+ *   (OakXxx *h, like av_frame_unref()/av_buffer_unref()) so it can
+ *   null out the caller's ctx after the final release; NULL and
+ *   ctx == NULL are no-ops. Out parameters that produce a handle
+ *   (e.g. option/positional-argument registration) also stay pointers.
+ */
+
+#endif //OAK_EDITOR_HANDLE_H