Ports the kon-llm model_manager resume pattern the rest of the way
into kon-transcription::model_manager:
- download() now validates an existing complete file against its
sha256 before skipping; a hash mismatch removes the file and
re-fetches, instead of serving a corrupt file to whisper.cpp.
- download_file() now distinguishes 206 Partial Content, 200 OK
(resume silently ignored by server), and other statuses, rather
than treating any non-206 as 'just use it as a fresh start'.
200-on-Range is handled by discarding the partial and starting
over cleanly.
- New tests: download_file_resumes_from_partial_and_verifies_sha
(TcpListener fixture, same shape as kon-llm's), and
download_file_fails_on_sha_mismatch_and_cleans_part_file.
- sha256_of_file helper + unit test for the existing-file guard.
Dev-deps: tempfile + tokio(net/io-util/macros). Total workspace
lib-test count: 116 → 123.
Co-authored-by: jars <jakejars@users.noreply.github.com>
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