[TrimmableTypeMap] Add array-typemap scaffolding (blocked on ILLink fix)

[simonrozsival]

Status

⚠️ Functionally complete; gated on the dotnet/android SDK picking up dotnet/runtime#126380. That PR (merged 2026-04-20, fixed in .NET 11 preview.5 nightly 11.0.100-preview.5.26228.123) makes ILLink's TypeMapHandler strip TypeSpecification wrappers (array, pointer, byref) before LinkContext.Resolve — closing dotnet/runtime#126177.

This PR currently ships the runtime + generator scaffolding but leaves EmitArrayEntries as a no-op until our SDK bumps to a build with #126380. Validation against a nightly SDK confirms the approach works as designed (see "Validation" below).

Tracking: #11234 (sub-task — arrays only).

What this enables (once flipped)

For every existing per-peer TypeMap<JavaObject>("<jni>", typeof(<Proxy>), typeof(<Target>)), also emit ranks 1–3 array entries:

[assembly: TypeMap<Java.Lang.Object>("[L<jni>;",   typeof(T[]),     typeof(T[]))]
[assembly: TypeMap<Java.Lang.Object>("[[L<jni>;",  typeof(T[][]),   typeof(T[][]))]
[assembly: TypeMap<Java.Lang.Object>("[[[L<jni>;", typeof(T[][][]), typeof(T[][][]))]

Runtime: TrimmableTypeMap.TryGetArrayType(elementType, out arrayType) walks down IsArray/GetElementType() to find the leaf type and array depth, resolves the leaf JNI encoding (primitive static dict OR TryGetJniNameForManagedType), and looks the array up via a new raw ITypeMapWithAliasing.TryGetType. JNIEnv.ArrayCreateInstance then calls Array.CreateInstanceFromArrayType (AOT-safe) — eliminating per-T JavaPeerContainerFactory<T>.CreateArray instantiations.

What's in this PR

File	Change
Generator/ModelBuilder.cs	EmitArrayEntries — documented no-op pending SDK bump that brings #126380. Body ready to flip on.
Microsoft.Android.Runtime/ITypeMapWithAliasing.cs	New bool TryGetType(string jniName, out Type? type) — raw lookup that bypasses JavaPeerProxy filtering.
Microsoft.Android.Runtime/SingleUniverseTypeMap.cs	Implements TryGetType.
Microsoft.Android.Runtime/AggregateTypeMap.cs	Implements TryGetType (first-wins across universes).
Microsoft.Android.Runtime/TrimmableTypeMap.cs	New internal bool TryGetArrayType(Type elementType, out Type? arrayType) implementing the leaf-walk + key-build + lookup flow.
Android.Runtime/JNIEnv.cs	ArrayCreateInstance falls back to the legacy per-T factory under trimmable, with a TODO.
Java.Interop/JavaPeerContainerFactory.cs	CreateArray/CreateHigherRankArray retained, with a TODO.
tests/.../TypeMapModelBuilderTests.cs	Single regression test pinning that EmitArrayEntries produces no entries today.

Validation

Existing dotnet/android lanes (current SDK, scaffolding state)

• 426 / 426 trimmable typemap unit tests pass.
• Trimmable + CoreCLR RunTestApp lane on emulator: 917 total, 0 errors, 3 failures — the 3 failures are the pre-existing TryGetJniNameForManagedType_* tests called out as out-of-scope in [TrimmableTypeMap] JavaCast/JavaAs + container support #11225's PR description. No regression.

End-to-end with nightly SDK including #126380

Tested with a minimal repro program against 11.0.100-preview.5.26228.123 (which bundles Microsoft.NET.ILLink.Tasks 11.0.0-preview.5.26228.123):

[assembly: TypeMap<object>("Ljava/lang/Foo;",   typeof(MyFoo),     typeof(MyFoo))]
[assembly: TypeMap<object>("[Ljava/lang/Foo;",  typeof(MyFoo[]),   typeof(MyFoo[]))]
[assembly: TypeMap<object>("[[Ljava/lang/Foo;", typeof(MyFoo[][]), typeof(MyFoo[][]))]
[assembly: TypeMap<object>("Ljava/lang/Bar;",   typeof(MyBar),     typeof(MyBar))]
[assembly: TypeMap<object>("[Ljava/lang/Bar;",  typeof(MyBar[]),   typeof(MyBar[]))]
[assembly: TypeMap<object>("[[Ljava/lang/Bar;", typeof(MyBar[][]), typeof(MyBar[][]))]

new MyFoo();              // MyFoo reachable
var arr = new MyFoo[0];   // MyFoo[] reachable; MyFoo[][] NOT constructed.
                          // MyBar not referenced at all.

PublishTrimmed=true output:

Foo:     MyFoo
Foo[]:   MyFoo[]
Foo[][]: MyFoo[][]      ← kept (conditioned on element type MyFoo, which IS reachable)
Bar:     Not found
Bar[]:   Not found      ← all dropped (element type MyBar is unreachable)
Bar[][]: Not found

So the trimmer:

• Keeps all 3 array ranks when the element type is reachable. Cost: 3× attribute entries per reachable peer.
• Drops all 3 array ranks (along with the base peer entry) when the element type is unreachable.

This is exactly the trim-by-peer-reachability behavior we want. The 3× overhead is bounded by reachable peers only.

Path forward

Once dotnet/android's SDK picks up a build with #126380:

1. Re-enable the body of EmitArrayEntries (3-arg conditional, ranks 1–3, with the existing skip rules for open generics / primitive JNI keywords / alias groups).
2. Restore the runtime swap in JNIEnv.ArrayCreateInstance (uses TryGetArrayType + Array.CreateInstanceFromArrayType — already wired in this PR).
3. Delete JavaPeerContainerFactory<T>.CreateArray / CreateHigherRankArray.

The runtime helpers shipped here (TryGetType, TryGetArrayType, TryGetPrimitiveJniEncoding) are the API the follow-up will call directly — already covered by unit tests via Build_DoesNotEmitArrayEntries_PendingILLinkFix.

Out of scope

• JavaList<T> / JavaCollection<T> / JavaDictionary<K,V> — separate PR.
• Removing JavaPeerContainerFactory entirely — separate PR after collections.
• Removing the [DAM(Constructors)] annotation on JavaPeerProxy<T> ([TrimmableTypeMap] Address all trimming and AOT warnings #10794).