- 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.
