feat: formalize P2 stress gates and ADR evidence traceability

[yianni]

Summary

• add reusable BackpressureStressHarness and StressInvariantTest to formalize stress invariants
• refactor stress validation runner to use harness and invariant checks
• add performance baseline report runner with machine-readable throughput/latency output
• strengthen CI with explicit stress invariant suite and performance baseline smoke run
• add ADR evidence matrix and P2 execution board docs; update README test/perf commands

Why

• completes P2 stabilization goals by making stress/perf validation explicit, repeatable, and traceable to ADR contracts

Validation

• sbt -Dsbt.supershell=false test passes (37 tests)
• sbt -Dsbt.supershell=false "Test / testOnly SimpleStreamProcessor.StressInvariantTest" passes
• sbt -Dsbt.supershell=false "Test / runMain SimpleStreamProcessor.BackpressureStressValidation 10000" passes
• sbt -Dsbt.supershell=false "Test / runMain SimpleStreamProcessor.PerformanceBaselineReport 5000 4 16" passes