Cap 1189 reset heartbeat on hidden

[daniel-graham-amplitude]

Summary

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Note

High Risk
Changes core event upload routing and queue semantics for delayed events, plus experimental video analytics event names and heartbeat-driven ingestion—mistakes could drop, duplicate, or mis-route production events.

Overview
Adds delayed event upload end-to-end: new delayedEventsServerUrl (US/EU defaults) and the destination plugin now batches events with delay.id to the delayed HTTP API, including payload translation (instant_events vs timed delayed), deduping superseded queue entries, and isFresh handling while a prior send is in flight.

Heartbeat now calls resetHeartbeat() when the document becomes hidden so delayed events flush sooner on background/hide.

Browser video capture is reworked to emit [Amplitude] Content Started / [Amplitude] Content Stopped via the shared client heartbeat (immediate start + long-timeout delayed stop that is updated during playback and flushed on real stop, teardown, or track failure). Playback treats waiting (buffering) as still active so sessions are not split; HTMLMediaElement / delivery_mode: audio are supported. VideoObserver infers waiting when the playhead stalls.

Tests and the test-server mock add coverage for delayed routing, stale events, visibility, and the new capture behavior.

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[linear-code]

CAP-1189

[github-actions]

size-limit report 📦

Path	Size
packages/analytics-browser/lib/scripts/amplitude-min.js.gz	66.66 KB (+0.24% 🔺)
packages/session-replay-browser/lib/scripts/session-replay-browser-min.js.gz	135.38 KB (+0.04% 🔺)
packages/unified/lib/scripts/amplitude-min.umd.js.gz	219.69 KB (+0.03% 🔺)
@amplitude/element-selector (gzipped esm)	3.4 KB (0%)