Add cross-variable preconditions for autoscale sizing constraints

[dmchaledev]

Summary

The autoscale tier variables document sizing invariants in their descriptions ("Must be between asg_min_size and asg_max_size") but didn't enforce them. A caller who sets asg_desired_capacity = 25 with asg_max_size = 20 gets a cryptic AWS provider error at apply time rather than a clear plan-time failure.

Terraform variable validation blocks can only reference the one variable being declared, so cross-variable constraints require a different mechanism. This PR uses terraform_data lifecycle precondition blocks (idiomatic Terraform ≥ 1.4, within our ≥ 1.5 floor) to add hard plan-time errors for three cross-variable invariants:

modules/unlimited-scale/aws/main.tf

• asg_min_size ≤ asg_desired_capacity ≤ asg_max_size — AWS rejects an ASG where desired is outside [min, max]
• db_max_allocated_storage_gb ≥ db_allocated_storage_gb — RDS autoscaling cap must be at least the initial allocation

modules/unlimited-scale/azure/main.tf

• vmss_min_count ≤ vmss_default_count ≤ vmss_max_count — Azure VMSS rejects a default count outside [min, max]

Error messages include the actual values so the caller can fix them without digging into provider logs:

Error: Resource precondition failed
  asg_desired_capacity (25) must be between asg_min_size (3) and asg_max_size (20).

Changes

• modules/unlimited-scale/aws/main.tf — adds terraform_data.validate_asg_sizing with two preconditions
• modules/unlimited-scale/azure/main.tf — adds terraform_data.validate_vmss_sizing with one precondition

No variables, outputs, or resources added. No behaviour change for valid inputs. The terraform validate and tflint CI gates will exercise the new HCL.

Test plan

• terraform validate passes in modules/unlimited-scale/aws and modules/unlimited-scale/azure
• tflint passes with no new findings
• Manually verify a plan with asg_desired_capacity outside [asg_min_size, asg_max_size] surfaces the precondition error at plan time
• Manually verify a plan with db_max_allocated_storage_gb < db_allocated_storage_gb surfaces the precondition error at plan time
• Default variable values (min=3, desired=3, max=20) pass all preconditions cleanly

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