feat(arch-layer,scripts): linux-gated desktop configs + package install scripts

niri (exact_config.d, scripts), noctalia (plugins excluded — runtime
state with embedded token), fontconfig, mimeapps.list,
niri-dynamic-workspaces, systemd user unit files (enable-state not
managed; only nirinit enabled by run_once), niri-launch vendored from
/usr/bin (unowned hand-written script, shellcheck warnings pre-existing).

Scripts: packages (pacman --needed + paru, brew on darwin, root-safe,
keyed on packages.yaml hash), gh extensions (idempotent), user services
(linux), post (tpm clone, bat cache, fc-cache). All shellcheck-clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

dot_config/systemd/user/nirinit.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=nirinit - Niri session save/restore
+PartOf=graphical-session.target
+After=graphical-session.target
+Requisite=graphical-session.target
+
+[Service]
+Type=simple
+ExecStart=%h/.cargo/bin/nirinit --save-interval 300
+Restart=on-failure
+RestartSec=2
+
+[Install]
+WantedBy=graphical-session.target