Add open-sticky window rule for sticky floating windows across workspaces

[AtefR]

Summary

• Add open-sticky window rule to keep a window visible across all workspaces on the current output
• Add toggle-window-sticky action to toggle stickiness via keybinds
• Add match is-sticky=true rule for matching sticky windows
• Expose is_sticky in IPC window info
• Treat sticky windows as floating for sizing/positioning rules
• Update docs and hotkey overlay

Testing

• cargo test -p niri
• Manual test: open a PiP window with open-sticky true, toggle stickiness via keybind, and verify it stays visible across workspace switches

Notes

• open-sticky implies open-floating
• Developed/implemented with assistance from OpenAI Codex; all changes reviewed and tested by me

[HigherOrderLogic]

From a quick skim, this looks like a less-featureful PR compared to #3205, which even supports pinning non-floating window.

[AtefR]

From a quick skim, this looks like a less-featureful PR compared to #3205, which even supports pinning non-floating window

the PR actually also allows non picture on picture or non-floating windows to be sticky too, my main objective was to be able to make a picture on picture window or a terminal sticky, I tested both and they're working flawlessly.

I also added the action toggle-window-sticky to allow binding it and is-sticky window matcher