Rename service replica groups to groups and support IP interpolation

[Bihan]

Services can now use the same groups: layout as tasks, and service commands can reference other replicas by IP with ${{ groups[i].replicas[j].IP_ADDRESS }}.

Previously, heterogeneous services used a list under replicas with count. That is renamed to groups with replicas (matching tasks’ groups / nodes). Old YAML still works; saved configs still use the old replicas: [{count}] layout.

A follow-up PR can deprecate that layout and save as groups instead.

Key changes

1. Both layouts work

Old

replicas:
  - count: 1
    commands: [...]

New

groups:
  - replicas: 1
    commands: [...]

Saved configs still use the old replicas layout, not groups.

2. Interpolation rules
   You can only reference replicas that are guaranteed at start:

- replicas: 2 → slots 0 and 1 are OK

- replicas: 1..4 → only slot 0 is OK

- replicas: 0..4 → no IP refs allowed

3. Preset fix
   Preset build had a helper that assumed service.replicas is a list of groups, but that list shape no longer exists in memory after this PR.

Before

resources = (
    service.replicas[group_num].resources
    if isinstance(service.replicas, list)
    else service.resources
)

After

for group in service.replica_groups:
    resources = group.resources
    resources.gpu.vendor = verification_vendor

This does not change behavior for presets that were already saved. Existing presets load as-is; this logic only runs when building a new preset.

Next: Separate PR for Docs