Add split view for side-by-side browser tabs

Summary

Display two or more existing Photon tabs side by side in one browser window.

Scope

• Keep pane topology, axis, and split ratios in framework-independent Rust state.
• Render panes through Photon’s existing native WebView and presentation lifecycle.
• Support creating a split, resizing dividers, moving tabs between panes, and collapsing a pane when its tab closes.
• Preserve split layout through session restore.
• Keep keyboard focus, navigation, and active-tab state clear for each pane.

Acceptance criteria

• Two or more tabs can be shown at once and resized without losing their page state.
• Closing or moving a tab leaves a valid layout.
• Split state survives window resize and session restore.
• Controls are keyboard accessible and do not reach into arbitrary Engine internals.

Design reference: Zephium models split layout as a pure pane tree with geometry and stable divider identity: https://github.com/zephium-browser/Zephium/blob/main/crates/zephium-core/src/split.rs