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8b8445f5ef fix(desktop): share one timer across same-interval useNow consumers (#5861 )

## Summary

Every `useNow(1000)` consumer owned its own `setInterval`. With dozens
of "agent working" surfaces mounted (sidebar channel badges, tray menu,
agent session panels, managed-agent rows), each ticked on its own
unaligned 1 s timer — a render/composite pass per consumer per second.
On a machine running ~23 agent sessions this pinned a sustained **~25%
of a core** in `com.apple.WebKit.WebContent` while the app sat idle.

This PR makes same-interval `useNow` consumers share one timer: all of
them tick in a single `setInterval` callback, so React batches the state
updates into one render pass. The last unsubscriber tears the timer
down; the visibility gate (pause while hidden, snap fresh on return) is
unchanged.

Attribution receipts (live dev build, 23 acp sessions): the shimmer was
the original suspect from `sample` stacks, but probing `animation: none`
left CPU flat (~25%), while clamping `useNow` intervals dropped it
immediately. Repeated A/B with this exact change: **~25% → ~3–9%**
webview CPU under the same agent load (ambient variance from live agent
activity; the delta reproduced across three alternations).

### Related issue

None found — follow-up to the presence-firehose investigation (#5830
fixed the subscription side; this is the remaining local render cost).

### Testing

- `pnpm test` — 4792/4792 pass, including a new test asserting N
same-interval consumers create exactly one timer and the last unmount
releases it
- `pnpm typecheck`, `biome check` — clean
- Live-local per TESTING.md: hot-patched into a running dev desktop with
23 active acp sessions; webview CPU dropped from ~25% sustained to ~3–9%
(A/B/A alternation, `ps` sampling over 30 s windows)

Signed-off-by: Eva <011987e296fd5006292d2f930b574be47c7801048d1983c46c425d3c95f0cffd@buzz.block.builderlab.xyz>
Co-authored-by: Eva <011987e296fd5006292d2f930b574be47c7801048d1983c46c425d3c95f0cffd@buzz.block.builderlab.xyz>

2026-08-14 10:16:10 -04:00