smp: reject duplicate STM short link updates

[pbottine]

Summary

• Reject STM LSET attempts that claim a LinkId already mapped to a different recipient queue.
• Keep same-queue link updates working.
• Add a regression that creates a victim short link, attempts to attach that link id to a separate authenticated queue, and verifies the victim link still resolves to the victim data.

Tests

• git diff --check origin/stable..HEAD
• cabal test simplexmq-test --extra-include-dirs=/opt/homebrew/opt/openssl@3/include --extra-lib-dirs=/opt/homebrew/opt/openssl@3/lib --test-option=--match --test-option='rejects attaching an existing short link to another queue'