docs(proposals): batched multi-kind node fetch design

Design doc for `client.get_many`: one GraphQL operation, one aliased
block per kind, one round-trip. Covers the primary surface, rejected
alternatives (DataLoader, lazy materialize, traversal-coupled), prior-art
comparison (DataLoader, selectinload, BatchGetItem, Apollo), internal
mechanism, demo collapse, back-compat, limitations, and a test plan.

Author: iddocohen