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docs/store/custom.md

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+# Custom Store
+
+Utility for creating custom Svelte stores with start/stop lifecycle and optional throttling.
+
+## Functions
+
+### custom(start, options)
+
+Creates a custom store that starts when first subscribed and stops when last subscriber unsubscribes.
+
+**Parameters:**
+- `start` - Function called when first subscriber is added. Receives a `set` function and should return an unsubscriber function
+- `options` - Optional configuration object
+
+**Options:**
+- `throttle` - Throttle subscriber notifications (milliseconds)
+- `onUpdate` - Callback function called when store value is set
+
+**Returns:** WritableWithGetter store with `get()`, `set()`, `update()`, and `subscribe()` methods
+
+## Example
+
+```typescript
+import {custom} from "@welshman/store"
+
+// Create a store that tracks window width
+const windowWidth = custom(
+  set => {
+    const updateWidth = () => set(window.innerWidth)
+
+    // Set initial value
+    updateWidth()
+
+    // Listen for resize events
+    window.addEventListener('resize', updateWidth)
+
+    // Return cleanup function
+    return () => window.removeEventListener('resize', updateWidth)
+  },
+  {
+    throttle: 100, // Throttle updates to every 100ms
+    onUpdate: (width) => console.log(`Window width: ${width}px`)
+  }
+)
+
+// Subscribe to changes
+const unsubscribe = windowWidth.subscribe(width => {
+  console.log("Width changed:", width)
+})
+
+// Get current value
+console.log("Current width:", windowWidth.get())
+
+// Clean up
+unsubscribe()
+```