Change screenshot capture ratio from 16:9 to 4:3 (litmus-dvjb)

Current screenshots are 1280x720 (16:9), which is too wide for terminal content — leaves dead space on the right. Switch to 4:3 (e.g. 960x720 or 1024x768) for a more natural terminal aspect ratio.

Tasks

• Decide exact capture dimensions: 80 cols × 32 rows → ~1280×960 (4:3 at 2x)
• Update capture config/code with new terminal window size
• Fix display resolution via wlr-randr (WLR_SCREEN_SIZE removed in wlroots 0.19)
• Re-capture all screenshots for both providers (kitty, wezterm)
• Rebuild manifest
• Verify screenshots look good at new ratio

Implementation

The root cause was that grim captures the entire Wayland display managed by cage, not just the terminal window. Changing TermGeometry rows only affected terminal cell config, not the compositor resolution.

Fix: Set WLR_SCREEN_SIZE environment variable on the cage command in capture.rs to {pixel_width}x{pixel_height} from the geometry. Added pixel_width and pixel_height fields to TermGeometry (default 1280x960 = 4:3).

Notes

• wlroots 0.19+ ignores WLR_SCREEN_SIZE env var — used wlr-randr --custom-mode inside the cage wrapper script instead
• Added pixel_width/pixel_height to TermGeometry (default 1280x960 = 4:3)
• Added wlr-randr to nix devshell dependencies

Re-capture requires Wayland + GPU environment:

mise run capture-kitty
mise run capture-wezterm
mise run capture-manifest

Then verify the new screenshots have ~4:3 ratio and look good.