v0.5.0

Minor release — observability phase 3a. Backward-compatible, still zero-dependency.

New

• Meter streaming-metrics hook (instrument-based, à la OpenTelemetry) plus
  WithMeter. The engine records produced / processed / emitted /
  consumed counters and a process_latency histogram on the hot path.
• Streaming (vs Observe's end-of-run snapshot) so it fits long-lived pipelines;
  both can be set in the same run. Opt-in — with no Meter the hot path pays
  nothing.

Boundary

Meter is stdlib-only, keeping the core zero-dependency. Concrete exporters
(Prometheus, OpenTelemetry) will land as separate submodules under flow/x/….

Notes

• The observability API (Observe, Report, Meter) stays experimental
  while v0.x.