The crypto engine was only reachable through the fcry binary; embedding
it in another Rust project meant shelling out to the CLI. Restructure
the crate so the binary sits on top of a proper library API.
- Add src/lib.rs exposing encrypt/decrypt/decrypt_range/derive_key, the
header and policy types, and the secret-handling primitives.
- Replace the positional-argument wrapper ladder
(encrypt_with_output_options, decrypt_with_argon_cap, ...) with
options structs: EncryptOptions, DecryptOptions, DecryptRangeOptions
and HeaderReadOptions. OutSinkOptions becomes the public
OutputOptions and no longer carries the input path; the input is now
an explicit parameter to OutSink::open_with_options so the
same-file-aliasing guard's inputs are visible at each call site.
- File parameters take Option<PathBuf>/&Path instead of AsRef<str>, so
non-UTF-8 paths work.
- FcryError implements Display and std::error::Error so it composes
with anyhow/thiserror-style error handling in downstream crates.
- Move read_key_file and normalize_passphrase from main.rs into
secrets.rs so library users get the same strict 32-byte key-file
parsing and NFC passphrase normalization. The world-readable
key-file warning stays in the CLI wrapper (read_key_file_cli).
- Drop now-unneeded #[allow(dead_code)] markers; ReadInfoChunk::Normal
loses its unused byte-count payload.
- Add rustfmt.toml (StdExternalCrate grouping, crate-granularity
imports) and reformat imports accordingly.
- Add tests/library_api.rs covering a file round-trip and a range
decrypt through the public API with a raw key.
User-visible change: CLI behavior is unchanged except error output,
which is now human-readable Display text ("Error: wrong key or
passphrase") instead of the Rust Debug representation.
Test plan: cargo clippy (default, --tests, --benches) is clean;
cargo +nightly fmt produces no diff; cargo test passes 43 tests
including the new library_api integration tests.
