feat(terminal): Kitty only — drop Ghostty (#95)

Ghostty's OpenGL 4.3 floor broke the Acer (HD 4000 / 4.2). Dual-terminal
support (default Ghostty + install-time Kitty fallback) was more complexity
than it was worth.

- Remove ghostty.nix / nomarchy.ghostty.enable
- Kitty always installed and themed from theme-state
- Default nomarchy.terminal = kitty; SUPER+Return, doctor, calendar,
  what-changed use kitty --class=com.nomarchy.*
- Drop install-time glxinfo probe and mesa-demos on the installer
- Docs/REQUIREMENTS no longer list OpenGL 4.3 as a terminal floor

Verified: V0 flake check; option-docs; installer-safety; template HM has
kitty, no ghostty; menu/calendar emit classed kitty launches.

docs/TESTING.md

@@ -47,7 +47,7 @@ nix build .#nixosConfigurations.nomarchy-live.config.system.build.isoImage
 
 The script prefers UEFI (OVMF) with a legacy-BIOS fallback, uses KVM when
 `/dev/kvm` is readable, and boots with `virtio-vga-gl` + `gl=on` —
-**Hyprland and Ghostty need real OpenGL in the guest**; without it the
+**Hyprland (and the desktop session) need real OpenGL in the guest**; without it the
 session may not start.
 
 ### What the live environment gives you
@@ -67,7 +67,7 @@ Work through these in order; each one exercises a different layer.
 |---|---|---|
 | 1 | Boots to Hyprland with the default theme wallpaper visible (Boreal) | greetd autologin, awww, session start |
 | 2 | Waybar shows at top, themed (blue accent on dark) | HM waybar module, palette baking |
-| 3 | `SUPER+Return` opens Ghostty with the default theme colors (Boreal) | terminal default, ANSI palette |
+| 3 | `SUPER+Return` opens Kitty with the default theme colors (Boreal) | terminal default, ANSI palette |
 | 4 | `btop` in the terminal is themed | per-theme asset baking |
 | 5 | `nomarchy-theme-sync list` prints 24 presets | package, baked themes dir |
 | 6 | `nomarchy-theme-sync apply gruvbox` → state written, `home-manager switch` runs, desktop re-themes, wallpaper changes | the whole engine: state write, pure eval, HM rebuild, wallpaper hook |