Add Cat plugin (#59)

An animated cat bar widget whose pace reflects CPU usage: sleeps
idle, walks under light load, runs under heavy load. Color follows
the theme by default or a pinned custom color. Cat artwork traced
from the MIT-licensed CatWalk plasmoid into a custom icon font.

cat/README.md

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+# Cat
+
+An animated cat that lives in your bar. It sleeps when your CPU is idle,
+walks as load picks up, and breaks into a full sprint under heavy load —
+colored to match your theme, or any color you pick.
+
+## Plugin
+
+| Field | Value |
+| --- | --- |
+| ID | `dotnetrob/cat` |
+| Entries | Bar widget: `cat` |
+
+## Usage
+
+Add the "Cat" widget to any bar from Settings → Bar → Add Widget. Click the
+widget to show a notification with the current CPU percentage.
+
+## Settings
+
+| Setting | Type | Default | Description |
+| --- | --- | --- | --- |
+| `cat_size` | `int` | `24` | Sprite size in the bar, in pixels (12–48). |
+| `show_cpu_percent` | `bool` | `false` | Display the CPU percentage next to the cat. |
+| `walk_threshold` | `int` | `15` | CPU percentage at which the cat wakes up and starts walking. |
+| `run_threshold` | `int` | `60` | CPU percentage at which the cat breaks into a run. |
+| `poll_interval` | `int` | `2` | How often to sample CPU usage, in seconds. |
+| `cat_color_mode` | `select` | `theme` | `theme` colors the cat with the palette's `secondary` role and tracks theme changes; `custom` uses the color below. |
+| `cat_color` | `color` | `#E8A24C` | Used when `cat_color_mode` is `custom`. |
+
+## Notes
+
+Every `poll_interval` seconds the widget reads `/proc/stat` to compute CPU
+usage — no other files are read or written, nothing is downloaded, and no
+processes are spawned. The cat's shape comes from a small custom icon font
+(`fonts/catwalk2.otf`) traced from the MIT-licensed
+[CatWalk](https://store.kde.org/p/2055225) plasmoid by Driglu4it, which lets
+it be recolored like normal bar text instead of a fixed-color image.