Harden authenticated gossip leases

[defcron]

Summary\n- replicate owner-signed registration, liveness, and record state rather than unauthenticated ownership hints\n- independently verify leases and expiry before adopting gossip\n- bind the five-name capacity rule to the canonical FCrDNS-verified host, rather than a free-to-rotate owner key\n- deterministically reconcile concurrently gossiped claims per verified host: retain the same first five everywhere and reject over-capacity claims without queueing or automatic later promotion\n- retain the winning signed lease on collision and derive a symmetric, token-seeded outcome so replicas converge\n\n## Validation\n- cargo test --workspace\n- cargo clippy --workspace --all-targets -- -D warnings\n- cargo fmt --all -- --check\n- cargo doc --workspace --no-deps\n- cargo llvm-cov --workspace --summary-only (98.38% line coverage)\n- git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 92.87305% with 32 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/binda-core/src/store.rs	88.63%	25 Missing ⚠️
crates/binda-core/src/gossip.rs	90.66%	7 Missing ⚠️

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