feat: isolated beat testing, headless pipeline, and mock telemetry harness

[IanEff]

Summary

Enables developers on resource-constrained laptops (or doing fast unit verification) to test and run thump's binaries in isolation without running a Kubernetes cluster, Docker/OrbStack VM, or the 12GB+ dev substrate.

What's included:

1. Golden Scenario Fixtures (test/fixtures/):
   • Canonical signal.Detection, proposal.Set, and decision.Governed payloads for cart-failure.
   • Prometheus and Loki response JSON payloads.
   • Comprehensive unmarshaling and schema invariant tests.
2. Isolated Beat Execution (internal/step):
   • Pure function entry points: step.RunRattle, step.RunClank, step.RunHiss, and step.RunThump.
   • Evaluates static JSON/YAML files and returns canonical domain objects without background daemons.
3. Headless Simulation Pipeline (internal/pipeline):
   • pipeline.Run chains detection -> proposal -> decision -> dry-run outcome in-memory in ~50ms.
4. Lightweight Telemetry & Broker Stubs (internal/mock):
   • mock.TelemetryServer: In-process HTTP server answering Prometheus PromQL and Loki query APIs.
   • mock.EmbeddedBroker: In-process NATS + JetStream server on ephemeral or configured ports.
5. Calipers Subcommands (cmd/calipers, internal/calipers):
   • calipers step <rattle|clank|hiss|thump>
   • calipers pipeline --detection <path>
   • calipers mock [--prom-port 9090] [--nats]
6. Taskfile Shortcuts & Docs (Taskfile.yaml, docs/dev-environment.md):
   • task beat:rattle, task beat:clank, task beat:hiss, task beat:thump
   • task dev:pipeline
   • task dev:mock
   • Complete documentation in docs/dev-environment.md.

Verification

• Full local CI passed: task ci (format check, vet, lint, vulncheck, kubeconform CRD/chart linting, promtool SLO tests, config validation, race tests, integration tests, and binary build).
• Verified unit and race tests across all new packages: internal/step, internal/pipeline, internal/mock, internal/calipers, and test/fixtures.
• Confirmed single-beat step execution and mock server on localhost.