docs: document the extensionworkers api

docs/extension-workers.md

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+# Extension Workers
+
+Extension Workers enable your [FrankenPHP extension](https://frankenphp.dev/docs/extensions/) to manage a dedicated pool of PHP threads for executing background tasks, handling asynchronous events, or implementing custom protocols. Useful for queue systems, event listeners, schedulers, etc.
+
+## Registering the Worker
+
+### Static Registration
+
+If you don't need to make the worker configurable by the user (fixed script path, fixed number of threads), you can simply register the worker in the `init()` function.
+
+```go
+package myextension
+
+import (
+	"github.com/dunglas/frankenphp"
+	"github.com/dunglas/frankenphp/caddy"
+)
+
+// Global handle to communicate with the worker pool
+var worker frankenphp.Workers
+
+func init() {
+	// Register the worker when the module is loaded.
+	worker = caddy.RegisterWorkers(
+		"my-internal-worker", // Unique name
+		"worker.php",         // Script path (relative to execution or absolute)
+		2,                    // Fixed Thread count
+		// Optional Lifecycle Hooks
+		frankenphp.WithWorkerOnServerStartup(func() {
+			// Global setup logic...
+		}),
+	)
+}
+```
+
+### In a Caddy Module (Configurable by the user)
+
+If you plan to share your extension (like a generic queue or event listener), you should wrap it in a Caddy module. This allows users to configure the script path and thread count via their `Caddyfile`. This requires implementing the `caddy.Provisioner` interface and parsing the Caddyfile ([see an example](https://github.com/dunglas/frankenphp-queue/blob/989120d394d66dd6c8e2101cac73dd622fade334/caddy.go)).
+
+### In a Pure Go Application (Embedding)
+
+If you are [embedding FrankenPHP in a standard Go application without caddy](https://pkg.go.dev/github.com/dunglas/frankenphp#example-ServeHTTP), you can register extension workers using `frankenphp.WithExtensionWorkers` when initializing options.
+
+## Interacting with Workers
+
+Once the worker pool is active, you can dispatch tasks to it. This can be done inside [native functions exported to PHP](https://frankenphp.dev/docs/extensions/#writing-the-extension), or from any Go logic such as a cron scheduler, an event listener (MQTT, Kafka), or a any other goroutine.
+
+### Headless Mode : `SendMessage`
+
+Use `SendMessage` to pass raw data directly to your worker script. This is ideal for queues or simple commands.
+
+#### Example: An Async Queue Extension
+
+```go
+// #include <Zend/zend_types.h>
+import "C"
+import (
+	"context"
+	"unsafe"
+	"github.com/dunglas/frankenphp"
+)
+
+//export_php:function my_queue_push(mixed $data): bool
+func my_queue_push(data *C.zval) bool {
+	// 1. Ensure worker is ready
+	if worker == nil {
+		return false
+	}
+
+	// 2. Dispatch to the background worker
+	_, err := worker.SendMessage(
+		context.Background(), // Standard Go context
+		unsafe.Pointer(data), // Data to pass to the worker
+		nil, // Optional http.ResponseWriter
+	)
+
+	return err == nil
+}
+```
+
+### HTTP Emulation :`SendRequest`
+
+Use `SendRequest` if your extension needs to invoke a PHP script that expects a standard web environment (populating `$_SERVER`, `$_GET`, etc.).
+
+```go
+// #include <Zend/zend_types.h>
+import "C"
+import (
+	"net/http"
+	"net/http/httptest"
+	"unsafe"
+	"github.com/dunglas/frankenphp"
+)
+
+//export_php:function my_worker_http_request(string $path): string
+func my_worker_http_request(path *C.zend_string) unsafe.Pointer {
+	// 1. Prepare the request and recorder
+	url := frankenphp.GoString(unsafe.Pointer(path))
+	req, _ := http.NewRequest("GET", url, http.NoBody)
+	rr := httptest.NewRecorder()
+
+	// 2. Dispatch to the worker
+	if err := worker.SendRequest(rr, req); err != nil {
+		return nil
+	}
+
+	// 3. Return the captured response
+	return frankenphp.PHPString(rr.Body.String(), false)
+}
+```
+
+## Worker Script
+
+The PHP worker script runs in a loop and can handle both raw messages and HTTP requests.
+
+```php
+<?php
+// Handle both raw messages and HTTP requests in the same loop
+$handler = function ($payload = null) {
+    // Case 1: Message Mode
+    if ($payload !== null) {
+        return "Received payload: " . $payload;
+    }
+
+    // Case 2: HTTP Mode (standard PHP superglobals are populated)
+    echo "Hello from page: " . $_SERVER['REQUEST_URI'];
+};
+
+while (frankenphp_handle_request($handler)) {
+    gc_collect_cycles();
+}
+```
+
+## Lifecycle Hooks
+
+FrankenPHP provides hooks to execute Go code at specific points in the lifecycle.
+
+| Hook Type  | Option Name                  | Signature            | Context & Use Case                                                     |
+| :--------- | :--------------------------- | :------------------- | :--------------------------------------------------------------------- |
+| **Server** | `WithWorkerOnServerStartup`  | `func()`             | Global setup. Run **Once**. Example: Connect to NATS/Redis.            |
+| **Server** | `WithWorkerOnServerShutdown` | `func()`             | Global cleanup. Run **Once**. Example: Close shared connections.       |
+| **Thread** | `WithWorkerOnReady`          | `func(threadID int)` | Per-thread setup. Called when a thread starts. Receives the Thread ID. |
+| **Thread** | `WithWorkerOnShutdown`       | `func(threadID int)` | Per-thread cleanup. Receives the Thread ID.                            |
+
+### Example
+
+```go
+package myextension
+
+import (
+    "fmt"
+    "github.com/dunglas/frankenphp"
+    frankenphpCaddy "github.com/dunglas/frankenphp/caddy"
+)
+
+func init() {
+    workerHandle = frankenphpCaddy.RegisterWorkers(
+        "my-worker", "worker.php", 2,
+
+        // Server Startup (Global)
+        frankenphp.WithWorkerOnServerStartup(func() {
+            fmt.Println("Extension: Server starting up...")
+        }),
+
+        // Thread Ready (Per Thread)
+        // Note: The function accepts an integer representing the Thread ID
+        frankenphp.WithWorkerOnReady(func(id int) {
+            fmt.Printf("Extension: Worker thread #%d is ready.\n", id)
+        }),
+    )
+}
+```