Performance tests

The performance tests are in crates/loci-core/tests/integration/perf.rs, #[ignore]d by default. They time import, the deck tree, Browse pages, a study answer, statistics and export on a large package, which is not committed: fixtures/large-100k.apkg, or the file named by LOCI_PERF_APKG. Without it the large-collection test skips.

LOCI_PERF_APKG=/path/to/large.apkg \
  cargo test --release -p loci-core --test integration -- --ignored perf --nocapture

Cold start is measured from process start to the app_ready command the interface sends when Home is interactive.