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/fontconfig/fonts.conf

@@ -0,0 +1,9 @@
+<?xml version="1.0"?>
+<!DOCTYPE fontconfig SYSTEM "urn:fontconfig:fonts.dtd">
+<fontconfig>
+    <match target="font">
+        <edit name="rgba" mode="assign">
+        <const>none</const>
+    </edit>
+  </match>
+</fontconfig>