feat(v2): add admin rotation

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Summary

Adds the same direct, current-admin-authorized set_admin mechanism to Tholos v2. Rotation takes effect immediately, emits AdminUpdated, and changes only who may invoke existing admin controls; pinned assertion policy and economics are unaffected. The v2 administration design documentation is updated.

Closes #129

Test plan

• cargo fmt --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test pass locally (make check)
• CONTRACT.md updated if the public interface changed — not applicable because it documents v1; V2_RESOLUTION.md is updated instead
• Fresh v2 WASM was built, deployed to testnet, and initialized from this branch
• Manually rotated the fresh deployment, confirmed the old admin was rejected by set_paused_v2, and confirmed the new admin was accepted