benchmarks: foundation runtime e2e baseline slice

[emil14]

Summary

This is part 1 (foundation) of splitting runtime benchmark baseline work from #1023 into small reviewable PRs.

This PR introduces the minimal e2e runtime benchmark setup needed before adding broader benchmark coverage.

Included in this slice

• Adds shared benchmark harness at benchmarks/bench_test.go.
• Replaces old single benchmark under benchmarks/message_passing/* with structured benchmarks/simple/* foundation cases:
  • simple/message_passing/int
  • simple/core/basic
• Exports minimal reusable test helpers in pkg/e2e/e2e.go for benchmark setup:
  • FindRepoRoot(testing.TB)
  • BuildNevaBinary(testing.TB, repoRoot)
  • PrepareIsolatedNevaHome(...)
  • CopyFile(...)
  • CopyDir(...)
• Makes benchmark discovery tolerant of partial rollout (simple without complex yet), so follow-up PRs can land incrementally.

Not included yet

• No benchmarks/complex/** cases.
• No expanded simple operator/type suites.
• No runtime benchmark CI workflow/docs additions.

Validation

• go test ./pkg/e2e ./benchmarks
• go test ./benchmarks -run=^$ -bench BenchmarkRuntimeE2E -benchtime=1x -count=1
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.5.0 run ./pkg/e2e ./benchmarks

Follow-up

Next PRs will add simple operators/types, then complex scenarios, then CI/docs.