Always on top: Add transparent support for on topped window (#44815)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
Transparency support (best-effort)
> Not every window can be made transparent. Transparency is applied on a
best-effort basis and depends on how the target app/window is built and
rendered.

## When it may not work
* Windows with special rendering pipelines (e.g., certain
hardware-accelerated / compositor-managed surfaces).
* Some tool/popup/owned windows where the foreground window isn’t the
actual surface being drawn.

## How it works (high-level)
* Resolve the best target window (preferring the top-level/root window
over transient children).
* Apply Windows’ standard layered-window alpha mechanism (per-window
opacity) to adjust transparency.
* When unpinned, Restore the original opacity/state when possible.

If transparency doesn’t change, it means the window doesn’t support this
mechanism in its current configuration.
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## PR Checklist

- [X] Closes: 
#43278 
#42929
#28773

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issues) -->
- [ ] **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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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed


https://github.com/user-attachments/assets/c97a87f2-3126-4e19-990f-8c684dbeb631

<img width="1119" height="426" alt="image"
src="https://github.com/user-attachments/assets/547671ee-81d3-4c94-8199-bf0c4b1b7760"
/>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/common/interop/shared_constants.h

@@ -72,6 +72,10 @@ namespace CommonSharedConstants
 
     const wchar_t ALWAYS_ON_TOP_TERMINATE_EVENT[] = L"Local\\AlwaysOnTopTerminateEvent-cfdf1eae-791f-4953-8021-2f18f3837eae";
 
+    const wchar_t ALWAYS_ON_TOP_INCREASE_OPACITY_EVENT[] = L"Local\\AlwaysOnTopIncreaseOpacityEvent-a1b2c3d4-e5f6-7890-abcd-ef1234567890";
+
+    const wchar_t ALWAYS_ON_TOP_DECREASE_OPACITY_EVENT[] = L"Local\\AlwaysOnTopDecreaseOpacityEvent-b2c3d4e5-f6a7-8901-bcde-f12345678901";
+
     // Path to the event used by PowerAccent
     const wchar_t POWERACCENT_EXIT_EVENT[] = L"Local\\PowerToysPowerAccentExitEvent-53e93389-d19a-4fbb-9b36-1981c8965e17";