Add WallPlotter p5 library with zigzag example.

Standalone package for Marlin polargraph control over Web Serial, including API docs and a runnable zigzag sketch.

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

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+# p5-wallplotter
+
+p5.js library for a Marlin polargraph (wall plotter) over Web Serial, plus a zigzag example.
+
+```
+p5-wallplotter/
+├── lib/
+│   ├── WallPlotter.js   # library
+│   └── README.md        # public API
+└── examples/
+    └── zickzack/        # example sketch
+        ├── index.html
+        └── sketch.js
+```
+
+## Run the example
+
+Chrome or Edge. Serve over localhost (Web Serial does not work from `file://`).
+
+```bash
+cd p5-wallplotter
+python -m http.server 8080
+```
+
+Open http://localhost:8080/examples/zickzack/
+
+Press **C** to connect, **H** to set home, then **Test** (preview) or **Plot** (send G-code).
+
+## Use in a sketch
+
+```html
+<script src="https://cdn.jsdelivr.net/npm/p5@1.11.3/lib/p5.min.js"></script>
+<script src="../../lib/WallPlotter.js"></script>
+<script src="sketch.js"></script>
+```
+
+```javascript
+let wp;
+
+function setup() {
+  createCanvas(windowWidth, windowHeight).parent('sketch-host');
+  wp = new WallPlotter();
+  wp.setSpeed(3000, 1500);
+  wp.enableKeyInputs();
+  wp.enablePlotButtons();
+}
+
+function draw() {
+  background(28, 28, 32);
+  wp.showInfos();
+}
+
+async function plot() {
+  await drawArt(false);
+}
+
+async function test() {
+  await drawArt(true);
+}
+
+async function drawArt(testOnly) {
+  wp.startPath();
+  wp.move(-200, -200);
+  wp.draw(200, -200);
+  await wp.runPath(testOnly);
+}
+```
+
+Full API: [lib/README.md](lib/README.md).
+
+Baud **115200**. Pen servo: `M280 P0 S90` up, `M280 P0 S50` down. Coordinates in mm, origin at home (`G92`).