perf(hot): avoid idle work on the SSE stream

[bjohansebas]

• skip payload serialization in publish() when no client is connected, and cut the ProgressPlugin callback early via eventStream.hasClients()
• start the heartbeat interval with the first client and stop it with the last one, instead of ticking forever
• pair bundles against the previous build through a name-grouped Map, replacing the per-bundle filter (O(n²) → O(n))

Summary

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Low Risk
Performance-only changes in dev-middleware HMR; no auth or production build paths, with behavior unchanged when clients are connected.

Overview
Reduces CPU and timer work when no browser is connected to the HMR SSE endpoint, while keeping behavior the same once clients are attached.

The event stream starts the heartbeat on the first client and stops it on the last disconnect (instead of running setInterval continuously), exposes hasClients(), and publish() returns early without JSON.stringify when clients.size === 0. With progress enabled, the ProgressPlugin callback bails out when there are no clients before publishing percent updates.

publishBundles now indexes previous bundles by name once and pairs by occurrence from that map, replacing a per-bundle filter over the full previous list (O(n²) → O(n) for many child compilers).

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