Fix headless screenshot capture: kitty EGL + config keys (litmus-haaf)

Two blocking issues preventing screenshot capture from working:

1. Wrong kitty config keys: initial_window_columns/initial_window_rows are not valid kitty config options (logged as ‘Ignoring unknown config key’). Correct keys are initial_window_width/initial_window_height (cell-count, no suffix).

2. EGL failure: kitty requires OpenGL/EGL but the wlroots headless backend (cage + WLR_BACKENDS=headless) does not expose EGL to client applications. kitty exits with ‘[glfw error 65542]: EGL: Failed to initialize EGL’.

Fix: Switch from cage+grim (Wayland headless) to xvfb-run+scrot (X11 virtual framebuffer). xvfb-run creates a virtual X11 display; scrot takes X11 screenshots. Mesa software rendering (LIBGL_ALWAYS_SOFTWARE=1) works on X11 Xvfb with no GPU.

Tasks

• Fix kitty config: initial_window_columns → initial_window_width, initial_window_rows → initial_window_height
• Switched approach: cage+grim with foot terminal (no OpenGL required) instead of xvfb+kitty
• Update flake.nix: added foot, grim, cage; foot config format fixed for v1.26.1
• Update screenshots.yml: uses nix devshell (cage+grim+foot), removed xvfb/mesa deps

Summary of Changes

• Root cause: Two separate issues blocked headless capture:

  1. Kitty config used initial_window_columns/initial_window_rows (invalid) → fixed to initial_window_width/initial_window_height
  2. Kitty requires OpenGL/EGL which wlroots headless backend doesn’t expose → switched to foot terminal (renders via Wayland SHM, no OpenGL)

• Architecture: cage (headless Wayland) + foot (SHM terminal) + grim (Wayland screenshot)

  • foot is pure Wayland, renders via shared-memory buffers — works perfectly with WLR_RENDERER=pixman
  • FootProvider generates foot 1.26.1 config with [colors-dark] section
  • cursor color uses double-value format: cursor = <bg> <text>

• Additional fixes:

  • Fixture command paths now canonicalized to absolute (was failing after cd to work dir)
  • FIXTURE_WORK_DIR exported to terminal environment
  • git-diff and git-log fixtures now use git --no-pager to prevent pager from blocking

• Verified: End-to-end capture of tokyo-night + git-diff and ls-color produces valid WebP screenshots