Benchmark Methodology

The fairness rules and the gate are in sbe/BENCHMARKS.md; how a pair is built, its observation barriers and the A/A control in sbe/benchmarks/methodology.md. Two rules to keep in mind:

• Treat a surprising ratio as a benchmark defect until wire parity, equal work, optimizer opacity, amplified timing, both LTO profiles and instruction counts agree.
• Compare against your own previous release as well as against sbe-tool. Two releases can both pass the sbe-tool ceiling while the second is slower.

Gate summary: Benchmarks.