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/94-portable-hdmi.kdl

@@ -0,0 +1,22 @@
+// ============================================================================
+// OUTPUTS
+// ============================================================================
+
+// Portable Display #1
+output "PNP(AOP) 16PM1QB 14490001D3X00" {
+    transform "90"
+    layout {
+        always-center-single-column true
+        center-focused-column "never"
+        gaps 2
+        default-column-width {
+            proportion 1.0
+        }
+        preset-column-widths {
+            proportion 0.5
+            proportion 1.0
+        }
+    }
+}
+
+