feat: add scoped metrics and cancellable iterator execution

[yianni]

Summary

• introduce pipeline-scoped metrics collectors via Metrics.Collector and thread-local collector context
• propagate scoped collectors through async execution handles, parMap workers, and async boundary producer threads
• extend ExecutionHandle with metricsSnapshot and add Node.runCancellableIterator(...)
• add Node-level recoverWith to align API surface with ADR recovery semantics
• strengthen CI stress gate by running calibrated backpressure validation at 10s

Why

• addresses P1 hardening goals: metric isolation, iterator cancellation parity, and stronger CI stress assertions

Validation

• sbt -Dsbt.supershell=false test passes (36 tests)
• sbt -Dsbt.supershell=false "Test / runMain SimpleStreamProcessor.BackpressureStressValidation 10000" passes