perf: visibility-gate dashboard render loop

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

src/decisions/DECISIONS_2026-02-16.md

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+# DECISIONS: perf-agent (2026-02-16)
+
+## Decision: Visibility Gating for Dashboard Refresh Loop
+
+- **Decision**: Modify the `setInterval` in `dashboard/index.html` to skip rendering if `document.hidden` is true.
+- **Rationale**: The dashboard currently re-renders the DOM every 30 seconds to update TTL bars, even when the tab is in the background. This wastes CPU cycles.
+- **Consequences**:
+  - Reduced CPU usage when tab is hidden.
+  - UI might be slightly stale when tab becomes visible, so we add a `visibilitychange` listener to force an immediate update.
+
+## Alternatives Considered
+- Remove the interval entirely: Would stop updates.
+- Use `requestAnimationFrame`: Still runs (sometimes throttled) and more complex to manage with 30s intervals.
+
+## Follow-ups
+- Monitor if users complain about stale data (should be mitigated by `visibilitychange`).