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/niri/noctalia.kdl

@@ -0,0 +1,35 @@
+layout {
+
+    focus-ring {
+        active-color   "#a398b3"
+        inactive-color "#1e1c21"
+        urgent-color   "#fd4663"
+    }
+
+    border {
+        active-color   "#a398b3"
+        inactive-color "#1e1c21"
+        urgent-color   "#fd4663"
+    }
+
+    shadow {
+        color "#1e1c2170"
+    }
+
+    tab-indicator {
+        active-color   "#a398b3"
+        inactive-color "#4a4158"
+        urgent-color   "#fd4663"
+    }
+
+    insert-hint {
+        color "#a398b380"
+    }
+}
+
+recent-windows {
+    highlight {
+        active-color "#a398b3"
+        urgent-color "#fd4663"
+    }
+}