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/exact_config.d/80-layer-rules.kdl

@@ -0,0 +1,34 @@
+
+
+
+/*
+  Noctalia v5: disable xray on all our surfaces so blur looks more realistic.
+  Noctalia publishes blur regions automatically when ext-background-effects is available.
+*/
+layer-rule {
+  match namespace="^noctalia-(bar-[^\"]+|notification|dock|panel|attached-panel|osd)$"
+  background-effect {
+    xray false
+    //blur false
+  }
+}
+
+// Option 2 (Stationary Wallpaper): place the regular wallpaper on the backdrop.
+layer-rule {
+  match namespace="^noctalia-wallpaper"
+  place-within-backdrop true
+}
+
+// Set transparent workspace background color so you see the backdrop at all times.
+layout {
+  background-color "transparent"
+}
+
+// Optionally, disable the workspace shadows in the overview.
+overview {
+  workspace-shadow {
+    off
+  }
+}
+
+