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/91-internal-display.kdl

@@ -0,0 +1,32 @@
+
+// Internal Display
+output "Tianma Microelectronics Ltd. TL134ADXP03 Unknown" {
+    position x=1920 y=0
+    scale 1.5
+    variable-refresh-rate on-demand=true
+    layout {
+        always-center-single-column true
+        center-focused-column "never"
+        gaps 8
+        default-column-width { proportion 0.9; }
+        preset-column-widths {
+            proportion 0.5
+            proportion 1.0
+        }
+    }
+}
+
+workspace "1" {
+    open-on-output "Tianma Microelectronics Ltd. TL134ADXP03 Unknown"
+}
+
+window-rule {
+    match app-id="org.gnome.Calendar"
+    match app-id="discord"
+    match app-id="slack"
+    draw-border-with-background false
+
+    open-focused true
+    open-maximized true
+    open-on-workspace "1"
+}