Post-demo hardening: define diamond ETH receive policy

[claude-do]

Track native ETH handling for the diamond.

Expected follow-up:

• Revert receive() if native ETH is unused, or add an explicit sweep/recovery policy.
• Add a focused test for the chosen behavior.

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