fix: revalidate vacuum links after row locking

[pathscale]

Summary

Fixes the intermittent versioned-publication failure observed on master in the WTI vacuum/upsert test.

Failed run: https://github.com/pathscale/WorkTable/actions/runs/30966635834

Vacuum snapshots reverse-index candidates before waiting for each row lock. A concurrent reinsert can move the primary key and retire or recycle the captured physical link during that wait. The old path trusted the captured link after locking, so it could move a different row and publish that row under the stale primary key.

This change:

• re-resolves the forward primary-index link after acquiring the row lock and mutation gate
• skips the candidate unless the captured link is still current
• uses LockGuard so lock and mutation-gate cleanup remains RAII-safe on errors
• adds a deterministic regression that moves the target key, recycles the stale slot for a different row, and verifies vacuum refuses the stale move

Verification

• Regression mutation check: disabling the new comparison makes the deterministic test fail
• Deterministic stale-link regression: passed
• Original red WTI vacuum/upsert test: passed 20 consecutive repetitions
• cargo test --workspace --all-targets --features versioned-row-publication: 515 passed, 0 failed, 4 ignored
• cargo clippy --all-targets --features versioned-row-publication -- -D warnings
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check

The Beta 5 release run later passed, confirming the failure is intermittent; the prior red exposed a concrete stale-link correctness race.