feat: gate synchronized page access

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Summary

• add an opt-in safe-concurrent-reads correctness mode
• place a table-wide read/write barrier around archived page-byte reads and mutations
• cover in-place updates, ghost publication, raw snapshots, page reuse, and vacuum row moves/resets
• compile the barrier out of default builds so the existing low-latency path is unchanged

Performance

Five alternating copied-binary benchmark pairs (30 samples, 0.5 s warm-up, 1 s measurement) found:

• simple_select_by_pk: +1.2% to +2.3%, median +1.5%
• simple_update: -0.9% to +1.0%, median +0.2% (noise-sized)

The enabled mode also intentionally serializes page mutations, so write-heavy concurrent workloads can see a larger throughput cost. The feature is therefore off by default.

Verification

• cargo test
• cargo test --features safe-concurrent-reads
• cargo clippy --all-targets -- -D warnings
• cargo clippy --all-targets --features safe-concurrent-reads -- -D warnings
• cargo fmt --check

The feature-gated regression test pauses publication after one field changes and verifies that a concurrent reader cannot pass the barrier or observe the incomplete row; after unghosting, it sees the complete row.

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Superseded by #187, which consolidated and merged the backend-selection and publication hardening work after the combined review and validation rounds.