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.

modules/home/stylix.nix

@@ -1,7 +1,7 @@
 # Stylix — themes the long tail of applications (GTK, Qt, cursors,
 # fonts) from the same theme-state.json that drives the live engine.
 #
-# Division of labour: the hot-reload trio (Hyprland, Waybar, Ghostty)
+# Division of labour: the hot-reload trio (Hyprland, Waybar, Kitty)
 # is owned by the Nomarchy engine and updates instantly; everything
 # Stylix touches updates on the next home-manager switch. That is why
 # autoEnable is off and the trio's Stylix targets stay disabled.