[Extensibility] Fix dual-path backend activation and Compile filtering

[PureWeen]

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Summary

This follows the GPT-5.6 Sol post-merge verification of #36654 and addresses two focused SingleProject gaps found against the merged implementation. It is part of #34099 and a follow-up to #36654; it does not close #34099.

Root causes and behavior changes

Dual-path backend identity

_MauiNormalizePlatformSpecificFolders only defaulted ActivationValue from BackendIdentity when both singular and plural target-platform metadata were empty. A registration that intentionally supported both paths, such as TargetPlatformIdentifiers="macos" BackendIdentity="macos", worked for a recognized macos TPI but could not activate on a neutral TFM through MauiActiveBackend=macos.

The default now applies whenever ActivationValue is empty, regardless of TPI metadata. Explicit activation values remain unchanged, explicit activation properties remain authoritative, and the existing neutral-path guard still prevents a backend selector from activating the folder on a different recognized TPI.

Cross-item Compile batching

_MauiRemovePlatformCompileItems created a fresh filesystem glob inside the ExcludeFromCurrentConfiguration=true batch. That glob also contained physical files represented by downstream Compile items explicitly marked false, so a separate true item could cause those active files to be removed.

The target now copies the actual Compile identities marked true to a temporary item type and intersects those identities with the platform filesystem candidates. It removes only the true identities without updating or re-adding active Compile items, preserving their metadata, ordering, and uniqueness.

Tests

Added real shipping-target regressions to MSBuildTests:

• one registration activates through both recognized macos TPI and neutral MauiActiveBackend=macos, while remaining excluded for recognized ios and a different neutral backend;
• two physical platform files in different metadata buckets retain only the explicit-false item, with exact order and no duplicates.

The two failures were reproduced before the target changes with direct workload-neutral MSBuild imports, then passed after the fixes. A 25-case direct shipping-target matrix passed for legacy shared folders, all five built-ins, GTK activation through ActivationValue and BackendIdentity, whitespace/unresolved fail-closed behavior, bare-activation compatibility, design-time unflip, ordering, and duplicate prevention.

dotnet format Microsoft.Maui.sln --no-restore --exclude Templates/src --exclude-diagnostics CA1822 --include src/Controls/tests/Xaml.UnitTests/MSBuild/MSBuildTests.cs --verify-no-changes completed successfully with reference-load warnings.

The repository-native filtered xUnit run and build-task solution build could not complete locally because this machine has .NET SDK 10.0.203 while global.json requires 11.0.100-rc.1.26379.102. The filtered test fails with NETSDK1045; the build first reports missing restore assets, and the allowed restore attempt confirms NETSDK1045 for the net11.0 projects.

Remaining limitations

CI with the pinned .NET 11 SDK is still required to execute the repository-native xUnit matrix and build. This PR remains scoped to SingleProject compile selection and does not implement the broader app-head manifest, entitlement, resource, launch, or design-time extensibility work tracked by #34099.

[github-actions]

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[Copilot]

Pull request overview

This PR adjusts SingleProject MSBuild targets to (1) default ActivationValue from BackendIdentity even when TargetPlatformIdentifier(s) metadata is also present (enabling dual-path activation), and (2) make _MauiRemovePlatformCompileItems remove only the Compile items explicitly marked ExcludeFromCurrentConfiguration=true rather than over-removing via cross-item-type batching.

Changes:

• Update _MauiNormalizePlatformSpecificFolders to back-fill ActivationValue from BackendIdentity whenever ActivationValue is empty (no longer gated by empty TPI metadata).
• Rework _MauiRemovePlatformCompileItems to compute the removal set by intersecting the platform filesystem candidates with the actual Compile items marked excluded.
• Add MSBuild regression tests covering (a) recognized vs neutral activation via BackendIdentity, and (b) compile-item removal behavior.

Show a summary per file

File	Description
src/Controls/tests/Xaml.UnitTests/MSBuild/MSBuildTests.cs	Adds regression coverage for compile removal correctness and dual-path backend activation.
src/Controls/src/Build.Tasks/nuget/buildTransitive/netstandard2.0/Microsoft.Maui.Controls.SingleProject.targets	Fixes platform Compile removal to avoid cross-item batching over-removal by intersecting with excluded Compile identities.
src/Controls/src/Build.Tasks/nuget/buildTransitive/netstandard2.0/Microsoft.Maui.Controls.SingleProject.Before.targets	Expands activation defaulting so a single registration can participate in both recognized-TPI and neutral-selector activation paths.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 0

[PureWeen]

Closing as superseded by #36956, which addresses the same two post-merge gaps with stronger regression coverage for Compile metadata preservation, ordering/uniqueness, and dual-path backend activation.