Benchmarks & ledger documentation

Benchmark methodology (QLM), running guides, ledger documentation, and performance investigations (flamegraph artifacts with question/environment/conclusion, REQ-BENCH-014).

Empty at M0 by design: methodology.md and running.md arrive at M2 with the harness; ledger.md at M5 with the runner (REQ-DOC-008).

Owner: MOD-BENCH, MOD-LEDGER.