feat: the Moonarch boot splash

Derived from tobilinuxer's 'Linux Distro Logos - macOS Boot Style'
(opendesktop.org/p/2106821), nixos-mac-style variant, under the X11
license. Renamed to moonarch in the descriptor's filename, its Name
entry and its ImageDir; the images are the original ones.

LICENSE

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+Copyright (c) tobilinuxer
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+Except as contained in this notice, the name of the copyright holder shall not
+be used in advertising or otherwise to promote the sale, use or other dealings
+in this Software without prior written authorization from the copyright holder.

README.md

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+# moonarch-plymouth-theme
+
+Plymouth boot splash for Moonarch. It runs on Plymouth's `two-step` module: a
+138-frame animation in the centre, a progress bar below it, and the password
+prompt Plymouth draws itself when the disk is unlocked.
+
+## Layout
+
+    moonarch.plymouth    theme descriptor, read by plymouthd
+    images/              animation frames and the prompt's own images
+    Screenshot.png       how it looks, from the original upload
+
+`ImageDir` in the descriptor names an FHS path. Packaging rewrites it to
+wherever the theme actually lands — see `pkgs/moonarch-plymouth.nix` in the
+moonix flake.
+
+## Origin
+
+Derived from *Linux Distro Logos - macOS Boot Style* by tobilinuxer
+(<https://www.opendesktop.org/u/tobilinuxer>), published at
+<https://www.opendesktop.org/p/2106821>, in its `nixos-mac-style` variant.
+Changed here: the theme's name, from `nixos-mac-style` to `moonarch`, in the
+descriptor's filename, its `Name=` entry and its `ImageDir`. The images are
+the original ones.
+
+The upload is under the X11 License, which keeps its copyright notice with the
+work — see `LICENSE`.