refactor: project structure (#9)

Split up project into multiple crates and use a Cargo workspace.

docs/sensor_mode2_fan.md

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+# Sensor Mode 2 Circular Progress
+
+A circular progress sensor (known as `fan` in the AOOSTAR-X software) masks a progress bar image for a certain angular
+range based on the corresponding sensor value. The masked image is alpha-blended with the panel image.
+
+Sensor configuration fields:
+- `mode`: 2 (for fan)
+- `direction`: 1 = clockwise, 2 = counter-clockwise
+- `label`: label identifier, also used as sensor value data source identifier
+- `x`, `y`: position on the panel
+- `width`, `height`: size of the circular progress element (not yet used)
+- `pic`: circular progress image to overlay. Should match `width`, `height`
+- `minAngle`, `maxAngle`: range of the masked image
+- `minValue`, `maxValue`: clamp sensor value to this range
+- `xz_x`, `xz_y`
+
+## Example
+
+The following configuration and graphics are taken from the `仪表盘_windows` panel configuration in `有线网卡 windows驱动.rar`.
+
+Example `panel.json` with a single "fan" indicator sensor and the following (partial) background image in `img`:
+
+<img src="img/sensor_mode2_background.jpg" alt="sensor mode 2 background image example">
+
+```json
+{
+  "name": "Fan test panel",
+  "img": "background.jpg",
+  "sensor": [
+    {
+      "id": "29d9ef2d-30b4-459d-b2b0-43cb6d4d6b41",
+      "itemName": "CPU usage",
+      "mode": 2,
+      "direction": 1,
+      "label": "cpu_percent",
+      "value": "47.7",
+      "x": 168,
+      "y": 184,
+      "width": 237,
+      "height": 237,
+      "minAngle": -160,
+      "maxAngle": 30,
+      "minValue": 0,
+      "maxValue": 80,
+      "xz_x": 0,
+      "xz_y": 0,
+      "pic": "progress_circle.png"
+    }
+  ]
+}
+```
+
+Progress image `"pic": "progress_circle.png"`:
+
+![progress graphic](img/progress_circle.png)
+
+The following graphic is rendered for progress example above:
+
+<img src="img/sensor_mode2.jpg" alt="sensor mode 2 example">
+
+
+## Known Issues
+
+Fan sensor rendering has been reverse engineered from the AOOSTAR-X app. Not all options are supported.
+
+- Work in progress, not yet fully tested
+- `direction: 2` doesn't seem to work
+- `widht`, `height` should be considered and auto-resized as for mode 4
+