keynote-2: add Postgres Rust client for apples-to-apples comparison

Add a Rust client for Postgres that mirrors SpacetimeDB's Rust client:
- Direct binary protocol via tokio-postgres (no HTTP, JSON, or Node.js)
- Multi-threaded Tokio runtime (one thread per connection)
- Batched/pipelined queries with prepared statements
- Calls do_transfer() stored procedure (single DB round-trip)

This eliminates all middleware overhead to isolate the genuine
architectural difference between SpacetimeDB and Postgres.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: zalo