Enable WAL mode for SQLite connections

[bcho]

Summary

• enable WAL journal mode when the extension initializes connections, tolerating in-memory databases that cannot switch modes
• add a unit test that verifies file-backed databases run in WAL mode by default

Testing

• cargo fmt -p absurd-sqlite-extension -- --check
• cargo clippy -p absurd-sqlite-extension -- -D warnings
• cargo test -p absurd-sqlite-extension

---

Codex Task