Function validate_swarm 

pub fn validate_swarm(spec: &DeploymentSpec) -> Result<(), ValidationError>

Validate pipeline-parallel inference swarm invariants.

Services that declare resources.gpu.sharding are grouped by swarm_id. For each swarm group this checks:

1. Every peers[].service reference resolves to a known service.
2. There is exactly one member with role: coordinator.
3. All members agree on layer_count.
4. The stage members form a contiguous [0, layer_count) cover with no gaps and no overlaps.

Errors

Returns a validation error if any of the above invariants is violated.