Standardize repository tooling, build configuration, and documentation

[rent-a-developer]

Description

Make the development and AI workflows consistent and easier to maintain. Preserve the public APIs, the localization behavior, the package identities, and the dependency floors.

• Separate the shared, source, and test build settings while preserving the generator, WPF, and package-wiring requirements. Validate the package contents and API compatibility against the last published version.
• Add dependency auditing so transitive advisories become visible.
• Align the C# style, member ordering, analyzers, and pinned tools. Use two spaces for all XML configuration. Preserve the XAML behavior.
• Triage the findings of the new analyzer set before the error gate is switched back on, so nothing is blanket-suppressed.
• Make every script check preserve the source and the Git index, so a check never rewrites the tree. Fixing stays an explicit action.
• Turn the packed-package Native AOT check into a real gate: publish and execute AvaloniaConsumer on .NET 8 and .NET 10 with zero IL2xxx and IL3xxx diagnostics. Today that job only checks that files exist.
• Align CI, quarterly Dependabot updates, contribution templates, AI instructions, focused reviewers, scoped hooks, and the commit and changelog rules.
• Keep the README short and put the longer material in docs/guides/ and docs/reference/. Move the DocFX configuration to build/docfx and the generated output to artifacts. The community health files stay at the repository root and are published from there.
• Apply the repository-wide formatting and ordering pass last, through the finished scripts.

The test runner stays on VSTest. A verification spike proved that a move to Microsoft.Testing.Platform does not work: Avalonia.Headless.XUnit fails with the newer xUnit, and the older xUnit cannot load any current Microsoft.Testing.Platform coverage extension.

Acceptance

• Every phase of the implementation plan is complete and its evidence is recorded.
• All four test suites run, and the packaging, native-consumer, and documentation checks pass.
• Shipping C# and XAML changes are mechanical only. The API, the dependency floors, the resources, and the runtime behavior are unchanged.
• Hook scope, non-mutating checks, documentation links, tool-version agreement, and local-package provenance are verified by hand.

Notes

The branch is merged with a merge commit, never squashed, because .git-blame-ignore-revs records the mechanical commit IDs.

Test-only C# and XAML changes are allowed. WPF Native AOT, benchmarks, and a design-decisions document are out of scope. The maintainer owns the branch, the release version, and publication. Do not add a permanent framework for the additional manual checks.