[O01.01.07.02] Implement GDM compile-time model generation from CLR types

[dotnetcadet]

Summary

Implement GDM compile-time model generation from CLR types, the AOT keystone. The walking-skeleton lands in W02 alongside G-01/G-02 so the Gdm runtime is built against the source-gen path, not the reflection path. Blocks G-07.

Acceptance Criteria

• A source generator produces GDM model metadata from CLR types.
• A walking-skeleton is usable by the Gdm runtime in W02.
• The generated metadata replaces reflection descriptors (feeds G-07).
• Tests cover the new behavior.
• The implementation remains NativeAOT-safe and trimming-safe.

Standards and Compliance

• Design decision D3 keystone: compile-time GDM metadata generation; blocks G-07.