Add default system groups for schedules

[waynemwashuma]

Objective

Introduce schedule-level default system groups so systems can automatically inherit a group when one is not explicitly specified.

Solution

Adds defaultSystemGroup support to Executable schedules and updates the scheduler builder to apply that group automatically during schedule construction.

Previously, every grouped system had to explicitly specify systemGroup, even when most systems belonged to the same phase. This created repetitive configuration and made schedule setup noisier.

The scheduler now resolves a system group using the following precedence:

1. Explicit systemGroup on the system
2. Schedule';s defaultSystemGroup
3. No group

The approach keeps grouping resolution centralized inside scheduler construction rather than mutating registrations during insertion, which avoids hidden side effects and keeps system registrations immutable.

Showcase

Before

scheduler.set(new Executable({
  label: 'update'
}))

builder.add({
  schedule: 'update',
  systemGroup: DefaultPhase,
  system: movement
})

builder.add({
  schedule: 'update',
  systemGroup: DefaultPhase,
  system: animation
})

After

scheduler.set(new Executable({
  label: 'update',
  defaultSystemGroup: DefaultPhase
}))

builder.add({
  schedule: 'update',
  system: movement
})

builder.add({
  schedule: 'update',
  system: animation
})

// Explicit groups still override the schedule default:
builder.add({
  schedule: 'update',
  systemGroup: AlternatePhase,
  system: networking
})

Migration guide

No migration required.

Checklist

• I have updated the documentation accordingly.
• I have added tests to cover my changes.