More progress.
I fleshed out mechanics a bit, not all of it is implemented but most is. It's a bit weird and goes as follows:
You're in a building and need to close all windows and doors to beat the game.
You have two depletable resources: soul and waiting time. When soul is exhausted the game is over.
Your soul diminishes by exposure to light. Each room in the building has a certain level of exposure that depends on the total area of room's open windows. When you close all windows in a room, the room becomes harmless.
Some rooms contain a Remnant. A Remnant is visible only if hit by direct light coming in through windows. If you close all windows in a room and a Remnant is in the room, it will instantly drain all of your soul and the game is over.
You can toggle the waiting mode where time runs faster and window light shifts across the room. This is used to search for a Remnant. Waiting time is a depletable resource.
You can gradually replenish soul by entering waiting mode in a room with all windows and all doors shut.