feat(terminal): themed Kitty when OpenGL < 4.3 (#95)

Ghostty needs OpenGL 4.3; older GPUs (Intel HD 4000) top out at 4.2.
Installer probes glxinfo and writes settings.terminal=kitty into
theme-state when below the floor. modules/home/kitty.nix enables Kitty
with the same palette, mono font, and opacity as Ghostty so the fallback
still looks like Nomarchy.

Default machines stay Ghostty-only (no kitty in the HM closure). Doctor
and calendar still invoke Ghostty by class — V3 on Acer for SUPER+Return.

Verified: V0 flake check; installer-safety; HM with terminal=kitty builds
themed kitty.conf + TERMINAL=kitty; default template has no kitty bin.

docs/REQUIREMENTS.md

@@ -37,13 +37,24 @@ that generation top out at OpenGL 4.2. No package upgrade on the distro can
 turn Ivy Bridge into a 4.3 device. Ghostty is choosing a modern GL baseline
 on purpose.
 
-**Until a fallback ships** (BACKLOG **#95** — alternate terminal or probe when
-GL < 4.3): machines below OpenGL 4.3 can still **install and run the
-desktop**, but SUPER+Return / the default terminal entry will fail until the
-user installs another terminal (e.g. kitty) or we ship an automatic fallback.
+**Install-time fallback (shipped):** when `nomarchy-install` can probe OpenGL
+(via `glxinfo` on the live session) and the version is **< 4.3**, it writes
+`settings.terminal = "kitty"` into the machine’s `theme-state.json`. Kitty is
+enabled and **themed from the same JSON** as Ghostty (palette, mono font,
+opacity). SUPER+Return then launches Kitty. Ghostty stays installed (doctor /
+calendar classed windows still call it — those helpers may still fail on old
+GL until a later follow-up).
+
+If the probe fails (no display / headless VM), Ghostty remains the default —
+do not guess. Existing installs can set the same by hand:
+
+```json
+"settings": { "terminal": "kitty" }
+```
+
+then `nomarchy-home` (or `home-manager switch`).
 
 Other Wayland clients vary; Hyprland itself is usually fine on these GPUs.
-The Ghostty floor is the one that bit real hardware first.
 
 ---