Refactor subnets mgmt in net-vpc-factory
This commit removes the ability to define subnets inline within the VPC `.config.yaml` across `net-vpc-factory` and enforces file-based subnet definitions using the `subnets/` subdirectory pattern.
Key changes include:
- **Module Updates:** Removed standard and non-standard inline subnet arguments (`subnets`, `subnets_private_nat`, `subnets_proxy_only`, `subnets_psc`) from the factory module's internal `vpcs` instantiation.
- **Schema & Docs:** Renamed `vpc.schema.json` to `vpc-factory.schema.json`, stripping inline subnet definitions from the JSON schema, and updated the module `README.md` to reflect the new file-based only approach.
- **Stage 2 Networking Configs:** Refactored datasets to migrate inline `subnets_proxy_only` definitions into standalone files like `subnets/prod-proxy.yaml`. Added a new exported output `subnet_ips` for downstream usage.
<!--
**Breaking Changes**
```upgrade-note
`fast/stages/2-networking`: Proxy-only subnets have been renamed for consistency - unfortunately this results in a nasty create-before-destroy which needs to be handled manually (i.e. delete the existing proxy-only-subnet first, either manually or with a targeted apply, then apply again)
```
-->
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