feat(api): add core marker to SubGroup

[SiorMeir]

Description

Adds an optional core marker to SubGroup (v2alpha2) so users choose which children of a SubGroupSet are part of a semi-preemptible PodGroup's core. Implements phase P1 of the HLD in kai-scheduler/KAI-Scheduler#2380.

• New SubGroup.Core bool (json:"core,omitempty"), CRD regenerated.
• PodGroup webhook rules (HLD "Validation"):
  • reject when core children under a parent exceed that parent's effective minSubGroup (unset = number of children; root uses the root minSubGroup);
  • reject adding, removing or moving core on update of a semi-preemptible PodGroup (not enforced when the update leaves semi-preemptible);
  • admission warning when core: true is set and preemptibility is not semi-preemptible (ignored; marks are not counted in that case);
  • redundant marks are accepted.
• Table-driven tests incl. nested trees and update immutability.

No scheduler behavior change here; that is phase P2.

Related Issues

HLD: kai-scheduler/KAI-Scheduler#2380

Checklist

• Self-reviewed
• Added/updated tests (if needed)
• Updated documentation (if needed)
• Added a changelog fragment via make changelog (or applied the skip-changelog label). Do not edit CHANGELOG.md directly — pending fragments are folded into it at release time.

Breaking Changes

None. The field is optional and ignored when unset.

Additional Notes

make generate, make manifests, make validate, make build, go test ./... pass. make clients was not needed (no new types). Follow-up: a KAI-Scheduler PR repins to this commit until a tag is cut.