Sample the browser monotonic clock with observation already subscribed, filter historical lifecycle and network facts, and preserve scope and termination handling when event time is unknown.
Bound pre-arm retention using transport-owned wire bytes. Fail closed when the relevant-event buffer fills, and keep unrelated protocol payloads outside that budget.
BREAKING CHANGE: observe_web_session timeline timestamps can be null when the producer supplies no comparable timestamp. Consumers must handle unknown time explicitly.
Refs #5609
morluto