Close unused SQLite connections

[lukewilliamboswell]

Summary

• give prepared statements shared ownership of their SQLite connection
• bound the ordinary path cache to 16 recently used connections while preserving sharing for live statements
• keep the exact :memory: path pinned per host thread and document that behavior
• close every non-null handle returned by a failed SQLite open
• cover failed opens, cache eviction, live statements, and in-memory reuse with unit tests

Verification

• cargo test --locked --lib
• cargo check --locked
• cargo clippy --locked --all-targets with existing generated-code lint exceptions
• ./scripts/test.py --no-build

Closes #435