perf-agent: visibility gate VideoEventBuffer

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

context/CONTEXT_2026-02-13_20-35-00.md

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+# Context
+
+## Goal
+Improve application responsiveness by optimizing background tasks and ensuring documentation accuracy.
+
+## Scope
+-   Analyze codebase for performance bottlenecks (timers, loops, concurrency).
+-   Fix: Visibility-gate `VideoEventBuffer` to prevent UI thrashing when hidden.
+-   Audit: Verify upload documentation matches implementation.
+
+## Assumptions
+-   `document.hidden` is reliable.
+-   `VideoEventBuffer` is the primary feed ingestion path.
+
+## Definition of Done
+-   `VideoEventBuffer` pauses UI updates when hidden.
+-   `daily-perf-report` generated.
+-   Lint passes.