Phase 0.6: Tiered OLAP benchmark harness

[thorrester]

Summary

• Adds tiered benchmark harness (Tier 0/1/2) gated by SCOUTER_BENCH_TIER env var, with JSON artifact output and bench_compare regression check
• Commits four Tier 0 baseline artifacts (bench_metrics/*.json) as the initial PR-blocking smoke baseline
• Wires Phase 0 observability through the full trace query stack: object store → Delta lifecycle → DataFusion pipeline → HTTP route handlers

Test plan

• make bench.core passes and artifacts match committed baselines
• make lints clean (clippy -D warnings)
• cargo test -p scouter-dataframe --all-features passes
• cargo test -p scouter-tracing --all-features passes (contract tests)