Support multiple aos-unit instances on one host

[klogg]

Problem / Motivation

Need to run multiple independent clusters on a single host (labs, CI, multi-tenant edge gateway).

Proposed behavior

• Introduce systemd template: aos-unit@.service
• Instance-scoped directories:
  • Runtime: /run/aos-unit/<instance>/...
  • Logs: /var/log/aos-unit/<instance>/...
  • State: /var/lib/aos-unit/<instance>/...
• Instance-scoped bridge naming and nftables table naming to avoid collisions.
• DNS domain suffix must be instance-aware (e.g. name.<instance>.aos-unit), or allow overriding suffix per instance.

Acceptance criteria

• Two instances can run concurrently without clobbering routes/nftables/dnsmasq leases.
• Clean shutdown of one instance doesn’t impact the other.
• Documented constraints and naming.