fix: preserve ordered overlapping durable writes

[pathscale]

Summary

• coalesce durable writes by physical page and offset so reused variable-length slots keep only the newest write
• apply overlapping writes in operation and queue creation order
• preserve ordering for rows sharing a multi-row operation ID
• allow batch metadata to represent multiple rows from one multi-row operation

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test --workspace
• AgencyZero: cargo test -p az-gui -- --test-threads=1 (187 passed)
• AgencyZero: cargo test -p wt-migrate (13 passed)

AgencyZero resolved worktable from /Users/revenge/code/WorkTable for the consumer verification. No live AgencyZero store was opened.