Migrate async runtime from async-std to tokio (required by libsql),
then add a mtime-based libsql cache at .nbd/cache.db that accelerates
nbd list, nbd ready, and nbd next by avoiding redundant file reads.
- Cargo.toml: replace async-std with tokio + libsql = "0.6"
- src/store.rs: async_std → tokio fs API; add open_cache() and
list_tickets_cached() with fallback to list_tickets on error
- src/main.rs: tokio::main, tokio::fs::remove_file; wire cmd_list,
cmd_ready, cmd_next to list_tickets_cached
- src/tests.rs: async_std::test → tokio::test, async_std::fs → tokio::fs
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Elijah Voigt