[A·M3a] Discovery — host sockets + Incus

[vxfemboy]

Milestone 3a (the discovery slice of epic #3).

• Host-socket scanner (pure /proc/net parser + reader)
• Incus instance + lifecycle-event parsing (user.blackwall.ports opt-in key)
• IncusClient trait + unix-socket adapter
• reconcile discovered services into the effective policy
• blackwalld run auto-opens/closes Incus-opted ports (detached event loop)

Delivered by PR #28. CAKE shaping (M3b) and banner/DNS fast-flux (M3c) remain under #3.