feat(menu): printer setup — system-config-printer in the System submenu

A `nomarchy-menu printers` entry opens system-config-printer (the CUPS
admin GUI), mirroring the bluetooth/blueman pattern: the printing service
ships the package via environment.systemPackages and the menu execs it,
self-gated on the system-config-printer binary so the entry appears only
when nomarchy.services.printing is on. No direct keybind (rare one-off).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/ROADMAP.md

@@ -85,6 +85,18 @@ how to override it. Items marked ✓ are shipped.
     so it'd mean adopting a new secret manager. Revisit if that changes.
   - Also: menu search is now **case-insensitive fuzzy** (`matching = fuzzy`,
     `sorting-method = fzf`) across every module + the launcher.
+- ✓ **Printer setup menu module:** a `nomarchy-menu printers` entry in the
+  **System** submenu opens **system-config-printer** (the CUPS admin GUI —
+  discovery, drivers/PPDs, options, test page), mirroring the bluetooth/blueman
+  pattern: the printing service ships the package (`environment.systemPackages`)
+  and the menu execs it, self-gated on the `system-config-printer` binary so
+  the entry appears only when `nomarchy.services.printing` is on. No direct
+  `SUPER+CTRL` bind — printer setup is a rare one-off, not a frequent utility
+  (left out deliberately; easy to add). Chose the GUI over a rofi-native
+  `lpadmin` flow (driver/PPD picking is impractical in rofi) and the CUPS web
+  UI (an unthemed browser page). Validated: the generated menu script's
+  `bash -n` build check passes, and an eval confirms printing-on puts
+  system-config-printer in `systemPackages`. Pending an on-machine check.
 - **Theme parity with legacy:** summer-day/night now carry their legacy
   bar layouts as `waybar.jsonc` whole-swaps (adapted: dead legacy script
   modules dropped, Nerd-Fonts-v2 codepoints remapped to FontAwesome/v3,