perf(wasm): Create image decode workers on demand - #25132

GitHub Issue: closes #25125

PR Type:

🐞 Bugfix (performance)

What changed? 🚀

Uno.UI.Runtime.ImageLoader created one decode worker per logical processor when the script loaded, whether or not the app ever decoded an image. Each worker is a separate isolate with its own heap and an OffscreenCanvas 2D context.

Workers are now created on demand. A decode uses an idle worker if there is one. Otherwise it creates a worker while fewer than navigator.hardwareConcurrency exist, and otherwise it waits in the existing queue. The limit is unchanged, so burst decoding keeps the same parallelism.

Measurements 📊

Master's SamplesApp on WASM in headless Edge, on a 12-thread machine. navigator.hardwareConcurrency was set through the DevTools protocol (Emulation.setHardwareConcurrencyOverride) before load, which sets the pool size without changing code. Memory is Windows private bytes of the tab's renderer process; the GPU process doesn't change with worker count. Two runs per row.

Idle cost, before this PR: about 4 MB per worker.

After the change, one worker exists at idle because the SamplesApp decodes an image at startup. An app that never decodes one creates none.

After a burst of 48 decodes of a 2048×2048 PNG, 15 s later:

• Each worker that decoded holds up to about 11 MB more. On master with 32 workers that's +347 MB.
• Its own GC releases this within about a minute: 971 MB at 15 s, 641 MB at 60 s.
• This PR doesn't change that peak once the pool has grown; idle release would be a follow-up.

Decode time for the same burst is unchanged:

• 12 workers: 794–844 ms on master, 802–817 ms with this PR.
• 4 workers: about 1.1 s on both.
• Throughput stops improving at about 6 workers on this machine.

Validation

• Runtime:
  • WASM Given_BitmapSource, in headless Edge: 16/16 pass, 1 skipped (an existing [Ignore]). This includes When_Browser_Workers_Are_Reused, which sends one more concurrent decode than hardwareConcurrency and so exercises growing the pool to its limit and reusing a worker.
  • The measurements above, run on the published app.
• Not added: a test for "no worker before the first decode". The pool is per process and earlier tests decode images, so the runtime-test app can't observe an untouched pool.
• Not measured: GPU memory of the workers' canvases on a real GPU. The headless browser here draws canvases in software.

PR Checklist ✅

• 🧪 Added Runtime tests, UI tests, or a manual test sample (for bug fixes / features, if applicable): see Validation.
• 📚 Docs have been added/updated following the documentation template (for bug fixes / features): not applicable.
• 🖼️ Validated PR Screenshots Compare Test Run results.
• ❗ Contains NO breaking changes
• 👀 Reviewed 2 other open pull requests (optional but appreciated!)

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