feat: add timing helpers and timing metrics dashboards

[streamer45]

Summary

• Introduced a core timing module with helpers for media clocks, metadata merging, duration math, and monotonicity; wired timing preservation through MoQ pull/push, WebM mux, pacer, and mixer.
• Added key OTel metrics for pacing, MoQ clock offset/inter-frame gaps, and pin distributor backlog (counts/bytes/seconds) plus pacer/engine/mixer refinements.
• Updated docs to describe new timing/backpressure metrics and expanded the Grafana sample dashboard with panels for pacer lateness/queues, MoQ timing, and pin distributor backlog/send-wait.