Reduce unsafe scope in trimmable type map code

[Copilot]

The trimmable type map path used broad unsafe patterns that do not align well with the upcoming C# memory-safety model. This revisits the generated/runtime registration path to make unsafe boundaries explicit while preserving existing performance-sensitive unsafe code where needed.

• Hashing helper

  • Preserves the existing stackalloc and pointer-based UTF-8 encoding path.
  • Keeps GetNamespaceAssemblyUtf8Bytes(...) unsafe and documents its caller requirements with /// <safety>.
  • Wraps calls to the unsafe helper in explicit unsafe blocks with SAFETY notes.

• Runtime native registration

  • Changed TrimmableTypeMap.RegisterNativeMethods() to remain safe-callable.
  • Scoped native pointer work to an inner unsafe block with a SAFETY note.

• Generated-code model

  • Updated the typemap emitter’s pseudo-C# to show the intended unsafe boundary pattern around JNI native registration.

Example pattern:

unsafe {
	// SAFETY: UTF-8 data is pinned for the duration of RegisterNatives.
	JniEnvironment.Types.RegisterNatives (runtimeClass.PeerReference, [method]);
}

[simonrozsival]

@copilot address this review

[simonrozsival]

I want to keep stackalloc, I don't want to compromise here. Simply use the unsafe block here instead.

[simonrozsival]

I want to keep this method the way it was. Keep it unsafe and just add the /// <safety> comment here + call it from unsafe { ... }

[Copilot]

@copilot address this review

Addressed the review in 9f6abc4 and follow-up cleanup 6252660. The stackalloc path is restored, GetNamespaceAssemblyUtf8Bytes remains unsafe with /// <safety>, and call sites now use explicit unsafe blocks.