[TrimmableTypeMap] JavaPeerScanner execution logic

Add JavaPeerScanner — the second phase that consumes AssemblyIndex
results and produces the final list of JavaPeerInfo for the app:

- Walks all indexed types and resolves their Java peer registrations
- Handles inheritance: traverses base types across assemblies to find
  the nearest registered Java peer ancestor
- Detects activation constructors (IntPtr+JniHandleOwnership) and
  distinguishes between direct declarations and inherited ones
- Collects marshal methods ([Register] on methods) and exported
  methods ([Export]) with their JNI signatures
- Merges component attribute metadata ([Activity], etc.) and resolves
  JNI names from attribute properties
- Flags types for unconditional preservation when component attributes
  with non-default JNI names are present

The scanner is designed as a pure function: assemblies in → peer info
out, with no side effects or global state.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: simonrozsival