Checkpoint. Known issues, fixed in part 2: short writes on stdout,
byte-wise stdin reads, Result suffix on all result schemas, lost enum
variant docs, serde errors as invalid params, missing item docs.
Imported from the spike branch, with:
- a library crate (Host API) for the CLI and the tests
- JSON test scenarios for candidates
- commit on close only for new or truncated files
- a Nix flake: nix run -- init, nix run -- run
surma