Implement bridges and fillout readme with current state

modules/networking/README.md

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+# Module: networking
+
+Creates the internal Linux bridge network segments on a Proxmox node. These bridges are purely virtual — no physical NIC is attached. All inter-segment traffic is routed through a firewall VM (OPNsense).
+
+## Segments
+
+| Bridge | CIDR | Purpose |
+|--------|------|---------|
+| management | 10.10.10.0/24 | Proxmox API access and admin tools. Proxmox host holds 10.10.10.1. |
+| services | 10.10.20.0/24 | General workload VMs and containers. |
+| dmz | 10.10.30.0/24 | Externally exposed workloads (e.g. web servers). |
+| isolated | 10.10.40.0/24 | Lab and test workloads. No outbound access by default. |
+
+The Proxmox host has no IP on services, dmz, or isolated — VMs on those segments have no direct path to the hypervisor.
+
+## Usage
+
+```hcl
+module "networking" {
+  source = "../../modules/networking"
+
+  proxmox_node_name = "pve"
+}
+```
+
+## Inputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| proxmox_node_name | string | Name of the Proxmox node to create bridges on. |
+
+## Notes
+
+- After apply, Proxmox automatically reloads the network configuration — no manual intervention required.
+- `Sys.Modify` must be granted to the Terraform API token role to manage node network interfaces.