changed interface to use instance methods rather than static ones

[virajmehta]

This will allow us to stick parameterized info into tasks so that we can e.g. execute code in a scripting language loaded at runtime.

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Note

Modernizes task API to be instance-based and enables dynamic registration.

• Change Task trait to use instance methods: name(&self) and run(&self, ...)
• Replace registry entries with Arc<dyn ErasedTask> via new TaskWrapper; worker clones handlers from registry
• Client updates: register<T: Task + Default>() now registers a default instance; add register_instance<T: Task>; type-safe spawn* now require Default and use the instance name()
• TaskContext::spawn<T> now requires Default and uses instance name(); telemetry field cleanup
• Public exports include ErasedTask and TaskWrapper; docs, benches, and tests updated with #[derive(Default)] tasks

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