Add clip parameter for viewport cropping

- Add clip object parameter to crop rectangular areas from screenshots
- Support POST body: clip {x, y, width, height} number fields
- Support GET query: clipX, clipY, clipW, clipH params
- Validation: all fields required, x/y >=0, width/height >0, max 3840x2160
- Mutually exclusive with fullPage and selector
- Update OpenAPI docs with clip examples
- Update Node.js and Python SDK READMEs with clip usage
- Add comprehensive test coverage (11 new tests)
- Tests: missing fields, negative coords, zero dimensions, max limits, mutual exclusivity

sdk/node/README.md

@@ -163,6 +163,28 @@ const complexCapture = await snap.capture({
 });
 ```
 
+### Crop Specific Areas (Clip)
+
+```typescript
+// Crop a specific rectangular area from the screenshot
+const croppedScreenshot = await snap.capture({
+  url: 'https://example.com',
+  clip: {
+    x: 100,      // X coordinate (pixels from left)
+    y: 50,       // Y coordinate (pixels from top)
+    width: 800,  // Width of the crop area
+    height: 600, // Height of the crop area
+  },
+});
+
+// Useful for capturing specific UI elements or sections
+const headerScreenshot = await snap.capture({
+  url: 'https://example.com',
+  clip: { x: 0, y: 0, width: 1280, height: 120 }, // Top banner only
+  format: 'png',
+});
+```
+
 ## API Reference
 
 ### `new SnapAPI(apiKey, config?)`
@@ -192,6 +214,7 @@ Returns a `Promise<Buffer>` containing the screenshot image.
 | `darkMode` | `boolean` | `false` | Emulate prefers-color-scheme: dark |
 | `hideSelectors` | `string \| string[]` | — | CSS selectors to hide before capture |
 | `css` | `string` | — | Custom CSS to inject before capture (max 5000 chars) |
+| `clip` | `object` | — | Crop rectangle: `{x, y, width, height}` (mutually exclusive with fullPage/selector) |
 
 ### `snap.health()`