Add a first-party schedule visualizer

## What problem does this solve or what need does it fill?

In traditional OOP-based game development, per-frame logic flow is generally easier to deduce because (usually) the call-stack *is* the schedule. We don't have that luxury with today's data driven scheduling mechanism.

## What solution would you like?

A way to visualize the schedule graph with all ordering constraints displayed. This could (and should) be part of the Bevy Editor when we get there, but a first-party solution now would also be nice.

### Recommended Features:

- Viewing individual Schedules.
- Searching by System Sets and Systems to see their scheduling constraints.
- See which Plugins registered which Schedules, System Sets, and Systems.
- Also extend this to registered hooks and observers, if possible.
- Live visualization of the schedule graph, with the ability to freeze the visualizer at any point for review.
- Breakpoint-style visualization freezes whenever a Schedule is modified.

Those last two will be very important for debugging if/when we get runtime schedule modification.

## What alternative(s) have you considered?

Continue using [`bevy_mod_debugdump`](https://crates.io/crates/bevy_mod_debugdump) as-is.

## Additional Context

Prior Art: [`bevy_mod_debugdump`](https://crates.io/crates/bevy_mod_debugdump)

Start by reviewing bevy_mod_debugdump, the cited prior art, and Bevy's scheduling and plugin concepts. Define the scope of a first-party schedule visualizer, including schedule-graph constraints, search, plugin provenance, and freeze behavior; done should be demonstrated by a working visualizer covering the agreed scope.

Written by the indexing model from the issue text.