Post-demo hardening: reject zero diamond owner in constructor

[claude-do]

Track diamond constructor hardening.

Expected follow-up:

• Require contractOwner != address(0) in the Diamond constructor.
• Add a focused constructor test.

Full triage doc: https://github.com/OmniPass-world/clan-world/blob/fix/diamond-init-finalize-guard/docs/reviews/pr468-gpt-5-5-pro-followup.md

Immediate fix PR: #475