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/20-layout-and-overview.kdl

@@ -0,0 +1,57 @@
+layout {
+    center-focused-column "never"
+    background-color "transparent"
+    default-column-width { proportion 0.5; }
+
+    preset-column-widths {
+        proportion 0.33333
+        proportion 0.5
+        proportion 0.66667
+    }
+
+    focus-ring {
+        width 1.5
+        active-color "#7fc8ff"
+        inactive-color "transparent"
+    }
+
+    border {
+        off
+        width 1.5
+        active-color "#ffc87f"
+        inactive-color "transparent"
+        urgent-color "#9b0000"
+    }
+
+    shadow {
+        off
+        softness 30
+        spread 5
+        offset x=0 y=5
+        color "#0007"
+    }
+
+    tab-indicator {
+        place-within-column
+        hide-when-single-tab
+        position "left" // Can be "left", "right", "top", or "bottom"
+    }
+
+
+}
+
+gestures {
+    hot-corners {
+    }
+}
+
+cursor {
+    xcursor-theme "capitaine-cursors-light"
+    xcursor-size 24
+    hide-when-typing
+}
+
+overview {
+    zoom 0.4
+    workspace-shadow { off; }
+}