Deduplicate UNIX and TCP server accept loops

[jeonghanlee]

Summary

The UNIX-server and TCP-server accept loops are near-identical (~45 lines each), differing only in the address struct and the client-name resolution. A fix to one must be mirrored to the other (maintenance entropy).

Location

con.cpp:659-706 (UNIX) / 747-800 (TCP).

Premise / fate

Both copies currently agree. Generalize (factor a shared accept loop) vs Keep — owner decision.

Verification

• T1 — if generalized, both server modes behaviorally identical pre/post
• both server paths green

Backlog (deferred). U5 is a symptom of this duplication. Register: docs/milestone.md. Tests land in the release cycle that schedules this.