fix: harden Database.close fail-fast behavior

[Helweg]

Summary

Harden Database.close() so native database handles enter an explicit closed state and fail fast on use-after-close instead of silently swapping to an in-memory SQLite connection.

Changes

• replaced the native Database connection storage with an explicit optional handle and shared helpers that return Database is closed after close
• made close() idempotent while preserving cleanup behavior by dropping the real SQLite connection instead of swapping in a replacement database
• added regression coverage for repeated close() calls, fail-fast post-close method calls, and file-release behavior after close

Testing

How were these changes tested?

• Unit tests added/updated
• Manual testing performed
• Build passes (npm run build)
• Typecheck passes (npm run typecheck)
• Tests pass (npm run test:run)
• Lint passes (npm run lint)

Release Labels

• Added at least one release category label (feature, bug, performance, documentation, dependencies, refactor, test, chore, or skip-changelog)
• Added at most one semver label (semver:major, semver:minor, semver:patch) when needed

Related Issues

Fixes #74