CmdPal: Add adaptive parallel fallback processing and consistent updates (#42273)

## Summary of the Pull Request

This PR replaces sequential fallback processing with an adaptive
parallel dispatch model and isolates fallback work onto a dedicated
thread pool, preventing misbehaving extensions from starving the .NET
ThreadPool on blocked synchronous COM RPC call.

The other major change is allowing MainListPage to allow take control
over the debounce of search updates directly, reducing latency and
improving smoothness of the search.

- Adds `DedicatedThreadPool` — an elastic pool of background threads
(min 2, max 32) that expand on demand when all threads are blocked in
COM calls and shrink after 30s idle.
- Extracts all fallback dispatch machinery (adaptive workers,
per-command inflight tracking, pending-retry slots) from MainListPage
into a standalone FallbackUpdateManager.
- Prevents one fallback from monopolizing all threads by capping
concurrent in-flight calls per fallback to 4.
- Starts with low degree of parallelism (2) and gently scales up to half
of CPU cores per batch. If a fallback takes more than 200ms, another
worker is spawned so remaining commands aren't blocked.
- Adds `ThrottledDebouncedAction` to coalesce rapid `RaiseItemsChanged`
calls from fallback completions and user input (100ms for external
events, 50ms adjusted for keystrokes), replacing unbatched direct calls.
- Bypasses the UI-layer debounce timer for the main list page since it
now handles its own throttling, eliminating double-debounce latency.
- Introduces diagnostics for fallbacks and timing hidden behind feature
flags.


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## PR Checklist

- [x] Closes: #42286
- [x] Related to: #44407
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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