Waybar fails to handle new niri IPC event `CastsChanged` (unknown variant)

### Description Waybar fails to parse niri IPC events after niri introduced a new event variant `CastsChanged`. This results in repeated errors and broken updates when using niri modules (e.g. workspaces). ### Logs Niri IPC error reading from event stream e=unknown variant CastsChanged, expected one of WorkspacesChanged, WorkspaceUrgencyChanged, WorkspaceActivated, WorkspaceActiveWindowChanged, WindowsChanged, WindowOpenedOrChanged, WindowClosed, WindowFocusChanged, WindowFocusTimestampChanged, WindowUrgencyChanged, WindowLayoutsChanged, KeyboardLayoutsChanged, KeyboardLayoutSwitched, OverviewOpenedOrClosed, ConfigLoaded, ScreenshotCaptured <img width="2519" height="236" alt="Image" src="https://github.com/user-attachments/assets/dcdb631a-fcb1-4c43-b235-24462ad9d34a" /> ### Steps to reproduce 1. Install latest niri (26.04+) 2. Install Waybar (stable, from pacman) 3. Enable niri-related modules in Waybar (e.g. workspaces) 4. Launch Waybar ### Expected behavior Existing IPC clients should not break when new event variants are introduced. Unknown events should ideally: - be ignored, or - not break the event stream ### Actual behavior Waybar fails to deserialize IPC events due to unknown variant `CastsChanged`, causing repeated errors and broken updates. ### System Information * niri version: 26 04 * Distro: Arch Linux * GPU: NVIDIA GeForce RTX 4060 Max-Q * CPU: Intel(R) Core(TM) i9-14900HX (32) @ 5.80 GHz ### Additional context This appears to be caused by adding a new IPC enum variant (`CastsChanged`) without backward compatibility. A possible improvement could be: - allowing unknown IPC variants to be ignored safely - or introducing IPC versioning / compatibility handling This would prevent breakage of existing ecosystem tools like Waybar.