Layer the remaining flake outputs onto the build: a default app pointing at
the compiled binary, a dev shell exposing the pinned Bun, biome, and bun2nix,
a `use flake` .envrc for direnv, and a checks set aggregating the Bun test
tiers, a build-seam smoke check against the Nix-built binary, and biome lint
through the treefmt-nix flake-parts module.
The dev shell and the build share the one pinned nixpkgs-bun input so Bun
cannot drift between them. treefmt's biome reuses the repo's biome.json with
VCS disabled for the sandbox, and packages/core/src/index_test.ts is
reformatted by biome so the new lint gate is green.
