Core Observable Architecture

Design and implement a generic `Observer` interface with `next`, `error`, and `complete` methods.

Implement a `Subscription` class with teardown logic for resource management.

Implement the core `Observable` class using a generic type parameter for the emitted value.

Implement the `subscribe` method to manage the connection between an `Observer` and the `Observable`'s producer function, returning a `Subscription`.

Write unit tests for the complete observable lifecycle: subscription, synchronous/asynchronous emission, error, completion, and unsubscription.