Hide the task handle's phantom context-data marker

The __contextData phantom field binds a TaskDefinition handle to its
federation's context data type, but as a string-keyed property it
leaked into user-facing docs and IDE completions despite its
@internal tag.  Replace it with a module-private unique symbol key:
no value exists at runtime, the marker disappears from completions,
and cross-federation handle rejection still type-checks, now guarded
by a regression test.

Also replace the tasks barrel's wildcard re-export of task.ts with
explicit named exports of the six types its consumers actually use,
so nothing new falls through the barrel unnoticed.

#803 (comment)

Assisted-by: Claude Code:claude-fable-5

Author: 2chanhaeng

packages/fedify/src/federation/tasks/mod.ts

@@ -7,4 +7,11 @@
  * @module
  */
 export { default as TaskCodec } from "./codec.ts";
-export * from "./task.ts";
+export type {
+  TaskDefinition,
+  TaskDefinitionInternal,
+  TaskDefinitionOptions,
+  TaskEnqueueOptions,
+  TaskHandler,
+  TaskRegistry,
+} from "./task.ts";

packages/fedify/src/federation/tasks/task.ts

@@ -77,6 +77,13 @@ export interface TaskDefinitionOptions<
   readonly queue?: MessageQueue;
 }
 
+/**
+ * Phantom key binding a {@link TaskDefinition} to its federation's context
+ * data type.  Declared only—no value exists at runtime, and the symbol is
+ * not exported, so the marker stays out of user-facing completions.
+ */
+declare const contextDataBrand: unique symbol;
+
 /**
  * The handle returned by {@link TaskRegistry.defineTask}.  It carries the
  * task name and schema so that {@link Context.enqueueTask} can validate the
@@ -101,7 +108,7 @@ export interface TaskDefinition<TContextData, TData> {
   /**
    * @internal Phantom marker binding the handle to its federation.
    */
-  readonly __contextData?: TContextData;
+  readonly [contextDataBrand]?: TContextData;
 }
 
 /**

packages/fedify/src/federation/tasks/tasks.test.ts

@@ -235,6 +235,14 @@ test("task type-level guards", () => {
     // @ts-expect-error: a wrong-shaped payload must not type-check.
     return ctx.enqueueTask(task, { n: "not a number" });
   };
+  const _crossContextHandleIsACompileError = (
+    ctx: Context<void>,
+    task: TaskDefinition<{ tenant: string }, { n: number }>,
+  ) => {
+    // @ts-expect-error: a handle bound to different context data must not
+    // type-check.
+    return ctx.enqueueTask(task, { n: 1 });
+  };
 });
 
 test("Context.enqueueTask() end-to-end", async (t) => {