main.rs was 683 lines — 95% of it was inference heuristics (get_job,
guess_from_filenames, get_compressor_from_filename, expand_shortcut_ext,
get_input_filename, get_path), Action, Job, and their unit tests. Move
all of that to a new job module, leaving main.rs at 96 lines containing
only the CmprssArgs/Format CLI shell and the thin command dispatch.

Behavior is unchanged; this is a pure reorganization commit.

FILE/DIRECTORY → File/Directory. Matches Rust's enum variant naming
convention and clippy's screaming_snake_case_ctor lint.

Every single-stream backend (gzip, xz, bzip2, zstd, lz4, brotli, snappy,
lzma) had the same ~40 lines of boilerplate per compress/extract for
resolving CmprssInput into a Read, resolving CmprssOutput into a Write,
and rejecting directory inputs/outputs. Consolidate that into a new
stream module and have each backend call open_input, open_output, and
guard_file_output.

Net -323 lines across the eight backends. Also normalizes the
directory-rejection error messages and extends the directory-input
guard to xz and bzip2, which previously only surfaced the lower-level
'Is a directory' error from File::open.

GITHUB_TOKEN events don't trigger downstream workflows, so the
release-plz workflow now dispatches publish.yml directly after
creating a release.

chore: release v0.3.0

Add MultiLevelCompressor that chains compressors together using
threaded pipelines for formats like tar.gz. Includes PipeReader/
PipeWriter for inter-stage streaming, Reader/Writer variants on
CmprssInput/CmprssOutput, and multi-level filename detection in
get_compressor_from_filename.