Extract network resources into reusable modules/network module

Moves the three incus_network resources into a dedicated module called
once per entry in var.incus_networks. State was migrated imperatively
via `terraform state mv` because lxc/incus v1.0.2 does not implement
ResourceWithMoveState, making declarative moved blocks unsupported.
Plan confirms zero creates/destroys after migration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

incus/network.tf

@@ -1,28 +1,8 @@
-resource "incus_network" "main_bridge" {
-  for_each = var.incus_hypervisors
-  name     = "main"
-  target   = each.key
-  type     = "physical"
-  config   = {
-    "parent" = "br0"
-  }
+module "network" {
+  source      = "./modules/network"
+  for_each    = var.incus_networks
+  name        = each.key
+  hypervisors = toset(keys(var.incus_hypervisors))
+  witnesses   = toset(keys(var.incus_witnesses))
 }
 
-resource "incus_network" "main_dummy" {
-  for_each = var.incus_witnesses
-  name     = "main"
-  target   = each.key
-  type     = "physical"
-  config   = {
-    "parent" = "dummy0"
-  }
-}
-
-resource "incus_network" "main" {
-  depends_on = [
-    incus_network.main_bridge,
-    incus_network.main_dummy
-  ]
-  name       = "main"
-  type     = "physical"
-}