tofu: bump k3s node memory_mib default 16384 -> 24576 (24 GiB)

Raise per-node RAM from 16 GiB to 24 GiB for proper failover headroom.
Matches the live rolling resize of the three k3s-dev VMs on Proxmox
(qm set --memory 24576 per node, one at a time, KSM enabled first).

Single memory_mib var applies to all three nodes via for_each, so a
future `tofu apply` would resize all three at once and break etcd
quorum — the resize MUST be done live/rolling, not via apply.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

infra/tofu/terraform.tfvars.example

@@ -36,7 +36,7 @@ ssh_authorized_key = "ssh-ed25519 AAAA...replace-with-your-public-key... user@ho
 
 # --- per-node sizing ---
 # vcpu          = 6
-# memory_mib    = 16384
+# memory_mib    = 24576
 # os_disk_gib   = 40
 # data_disk_gib = 100
 # cpu_type      = "host"

infra/tofu/variables.tf

@@ -83,9 +83,9 @@ variable "vcpu" {
 }
 
 variable "memory_mib" {
-  description = "RAM per node in MiB (16 GiB)."
+  description = "RAM per node in MiB (24 GiB)."
   type        = number
-  default     = 16384
+  default     = 24576
 }
 
 variable "os_disk_gib" {