Replaces the hand-rolled step rescheduling loop in `Model` with an internal `EventGenerator`, simplifying the code while keeping all behavior and API identical.

The current step scheduling uses a manual pattern: `_do_step()` calls `_schedule_step()`, which creates a `SimulationEvent` that calls `_do_step()` again. This is functionally an `EventGenerator` — but reimplemented ad-hoc. Now that `EventGenerator` exists (#3201), we should use it.

This also removes `_schedule_step()` entirely and simplifies both `_do_step()` and `_wrapped_step()`, reducing the amount of scheduling logic in `Model`.

### Summary
Exposes event scheduling and time advancement methods directly on `Model`, giving users a public API for scheduling events and advancing simulation time without needing to interact with the `Simulator` class.

Basically, these methods allow us:
- to replace all existing behavior (in examples, tutorials, etc.)
- start deprecation all current scheduling and stepping APIs
- remove them in Mesa 4.

### Motive
Currently, users advance their models by calling `model.step()`, which is tightly coupled to the concept of a single discrete step. With the event-based architecture taking shape (#3201, #3204), we need public methods that let users think in terms of *time* rather than *steps*.

The key method here is `model.run_for(duration)`. For existing ABM users, `model.run_for(1)` is functionally equivalent to `model.step()`: it advances time by 1 unit, which triggers the scheduled step event. But unlike `step()`, it generalizes naturally: `run_for(0.5)` advances half a time unit, `run_for(100)` advances 100 units processing all scheduled events along the way. This makes `run_for` the foundation for both traditional ABMs and event-driven models.

This is also a stepping stone toward deprecating `model.step()` as the primary way to advance a model. Instead of calling a method that *is* a step, users call a method that *advances time*, and steps happen as scheduled events within that time. The mental model shifts from "execute step N" to "advance time, and whatever is scheduled will run."