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/scripts/executable_kill-phantom-display.fish

@@ -0,0 +1,20 @@
+#!/usr/bin/env fish
+# Turn off the phantom Studio Display leg (DP-4/5/6, varies per boot).
+# Its signature is a single 640x480 mode — connector name and EDID are unreliable.
+# ponytail: one-shot with retry at startup; doesn't handle mid-session hotplug —
+# switch to a `niri msg event-stream` watcher if that starts biting.
+for i in (seq 10)
+    set -l phantoms (niri msg -j outputs 2>/dev/null | jq -r '
+        to_entries[]
+        | select((.value.modes | length) == 1
+                 and .value.modes[0].width == 640
+                 and .value.modes[0].height == 480)
+        | .key')
+    if test -n "$phantoms"
+        for out in $phantoms
+            niri msg output $out off
+        end
+        exit 0
+    end
+    sleep 1
+end