perf-agent: visibility gate VideoEventBuffer

Co-authored-by: PR0M3TH3AN <[email protected]>

docs/agents/task-logs/daily/2026-02-13_20-35-00_perf-agent_started.md

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+Started perf-agent run for 2026-02-13.

docs/agents/task-logs/daily/2026-02-13_20-45-00_perf-agent_completed.md

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+# Completed: perf-agent
+
+**Timestamp:** 2026-02-13 20:45:00
+**Agent:** perf-agent
+**Summary:**
+Implemented visibility gating for `VideoEventBuffer`. Now pauses UI updates (flushes) when the tab is hidden, accumulating changes to be applied in a batch when visibility is restored. This reduces background CPU usage and UI thrashing. Verified upload documentation against code (2GB limit is accurate). Generated daily perf report.