test: pin multi-row primary-key lock order

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Summary

• Add a deterministic regression test for generated multi-row update lock ordering.
• Construct a non-unique index order whose first row is not primary key zero.
• Hold that index-first row and verify the generated update registers the primary-key-zero lock before it blocks.
• Verify the update resumes and applies uniformly after the predecessor lock is released.

Regression proof

• Passes on current master, which snapshots, sorts, and deduplicates primary keys before locking.
• Fails when applied to 9c0d98f^ with: the update followed secondary-index order instead of locking primary key zero first.

Performance

• Test-only change. No production code or generated hot path changes.

Verification

• cargo test --test mod worktable::lock_order::multi_row_update_locks_in_primary_key_order_not_index_order
• cargo clippy --all-targets
• cargo fmt --all -- --check

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Superseded by #187, which consolidated and merged the backend-selection and publication hardening work after the combined review and validation rounds.