Replace trimmable manifest LINQ XML generation with lightweight XML model

[Copilot]

The trimmable typemap manifest path used System.Xml.Linq for template parsing, read/modify/write merging, rooting, and final output. This PR moves production manifest generation to a lower-allocation XML model while preserving existing manifest behavior.

• Manifest model

  • Adds a small ManifestDocument/element/attribute model.
  • Parses templates with XmlReader.
  • Writes merged manifests with XmlWriter.

• Generation pipeline

  • Updates GenerateTrimmableTypeMap, TrimmableTypeMapGenerator, and GeneratedManifest to pass ManifestDocument instead of XDocument.
  • Keeps output flowing through Files.CopyIfStreamChanged(...).

• Manifest merge helpers

  • Converts component, assembly-level, and property mapping helpers from XElement mutation to lightweight model mutation.
  • Preserves placeholder replacement, compat-name rewriting, duplicate detection, runtime provider generation, application attributes, and assembly-level manifest elements.

• Rooting

  • Roots manifest-referenced peers from the lightweight model instead of walking an LINQ-to-XML DOM.

Example API shift:

ManifestDocument? manifestTemplate = null;
if (!ManifestTemplate.IsNullOrEmpty () && File.Exists (ManifestTemplate)) {
	manifestTemplate = ManifestDocument.Load (ManifestTemplate);
}

[simonrozsival]

@copilot build fails:

...

       "/mnt/vss/_work/1/s/android/Xamarin.Android.sln" (default target) (1:2) ->
       "/mnt/vss/_work/1/s/android/src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj" (default target) (11:10) ->
       (CoreCompile target) -> 
         /mnt/vss/_work/1/s/android/src/Xamarin.Android.Build.Tasks/Tasks/GenerateTrimmableTypeMap.cs(149,41): error CS0117: 'ManifestDocument' does not contain a definition for 'Load' [/mnt/vss/_work/1/s/android/src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj]
         /mnt/vss/_work/1/s/android/src/Xamarin.Android.Build.Tasks/Tasks/GenerateTrimmableTypeMap.cs(158,23): error CS1503: Argument 6: cannot convert from 'Xamarin.Android.Tasks.ManifestDocument' to 'Microsoft.Android.Sdk.TrimmableTypeMap.ManifestDocument?' [/mnt/vss/_work/1/s/android/src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj]

    200 Warning(s)
    2 Error(s)

[Copilot]

@copilot build fails:

...

...

Fixed in e6721fd by aliasing the trimmable manifest model type in GenerateTrimmableTypeMap, so it no longer resolves to Xamarin.Android.Tasks.ManifestDocument. Verified with dotnet build src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj --no-restore -p:BuildProjectReferences=false and the trimmable typemap test project.