feat(benchmarks): add benchmark report, fix gitignore and README

- Add benchmarks/reports/REPORT_ONIX_v150.md — baseline report
  (Apple M5, darwin/arm64, Beckn v2.0.0, GOMAXPROCS=10)
- Gitignore benchmarks/results/ — runtime output from run_benchmarks.sh
- Update README: directory layout with reports/ vs results/, Reports
  section with workflow for adding new reports, fix benchstat invocation
  to use `go tool benchstat`
- Remove internal task marker from setup_test.go comment

benchmarks/e2e/setup_test.go

@@ -269,7 +269,7 @@ func buildBAPCallerConfig(routingConfigPath, registryURL string) module.Config {
 	}
 }
 
-// ── T7: Request builder and Beckn signing helper ──────────────────────────────
+// ── Request builder and Beckn signing helper ─────────────────────────────────
 
 // becknPayloadTemplate holds the raw JSON for a fixture file with sentinels.
 var fixtureCache = map[string][]byte{}