Fix default in-memory pool isolation and add shared-memory pools

[ryanfowler]

Summary

• Default anonymous in-memory pools to a single connection so PoolBuilder::new().open() and path(":memory:") no longer round-robin across isolated databases.
• Reject explicit multi-connection anonymous memory pools with a config error.
• Add PoolBuilder::shared_memory(name) for intentional multi-connection shared in-memory databases using SQLite URI + shared-cache flags.
• Update docs and AGENTS guidance to describe the new pool behavior and SQLite caveats.

Testing

• Added regression coverage for default in-memory pools, explicit :memory: pools, config errors, and named shared-memory pools.
• cargo test passed.
• cargo fmt --all -- --check passed.
• cargo clippy -- -D warnings passed.