refactor(bench): migrate traversal and MERGE scaling measurements to Divan

[DecisionNerd]

Summary

• Add Divan bench targets traversal_scaling and merge_scaling with warmed traversal fixtures, per-hop workloads, and fresh MERGE samples (timed region includes filler construction; documented boundary change).
• Retain deterministic product CI gates: io_smoke / I/O scaling assertions in bench_traversal_scaling.rs and topology-read bound in merge_scaling_bench.rs (no custom timers).
• Update make bench-traversal, add make bench-merge-scaling, refresh measurement inventory and benchmarking docs.

Stacks on #1486 (closes #1484). Closes #1485.

Test plan

• cargo test -p graphforge-exec --test bench_traversal_scaling io_smoke
• cargo test -p graphforge-exec --test merge_scaling_bench node_merge_topology_read_count_is_bounded_not_per_row
• cargo bench -p graphforge-exec --bench merge_scaling -- --sample-count 1 (tiny smoke)
• python3 scripts/ci/benchmark-measurement-policy.py
• python3 scripts/ci/cargo-bazel-drift-check.py
• CI Gate after test(bench): define and enforce BenchExec/Divan measurement policy #1486 merges and this PR retargets to main (or green stacked CI)

Made with Cursor

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