feat: add multi-subnet support for CoreDHCP/coresmd

[sujit-jadhav]

Summary

Add optional additional_subnets configuration under admin_network in network_spec.yml to support multi-RAC / multi-subnet PXE deployments with CoreDHCP relay (giaddr-based routing). Requires coresmd with multi-subnet support (coresmd PR #61).

Changes

File	Change
input/network_spec.yml	Added additional_subnets: [] field with documentation
.../schema/network_spec.json	JSON schema validation for additional subnet entries
.../en_us_validation_msg.py	6 error messages for subnet validation
.../provision_validation.py	_validate_additional_subnets() + _ranges_overlap() — validates CIDRs, routers within subnet, ranges within subnet, no overlap with admin or between subnets, no DHCP pool overlap
.../templates/configs.yaml.j2	Emits coredhcp_subnets and coredhcp_subnet_pools lists
.../templates/coredhcp.yaml.j2	New — dual-mode CoreDHCP template
.../vars/main.yml	Template path variables
.../tasks/deploy_openchami.yml	Copy coredhcp template after clone
.../tests/test_additional_subnets_validation.py	New — 17 unit tests

Backward Compatibility

additional_subnets	coresmd version	Config format	Works?
[] (single subnet)	v0.4.x (current)	Positional args (original)	✅
[] (single subnet)	New (PR #61)	Positional args (backward compat)	✅
Populated (multi-subnet)	New (PR #61)	key=value with subnet=/subnet_pool=	✅

Testing

• 37 unit tests pass (17 new + 20 existing)
• Validation covers: invalid CIDR, router outside subnet, range outside subnet, admin overlap, inter-subnet overlap, DHCP pool overlap, invalid netmask, invalid range format
• Single-subnet generates byte-identical config to original template