fix: shasum -c / sha256sum -c read checksum lines from stdin

[davydog187]

Fixes #81

shasum -c / sha256sum -c treated - as a filesystem path and treated a missing operand as success after verifying nothing. That is a silent wrong answer for agent tool-use: a caller checking a checksum got exit 0 whether or not the file matched.

GNU coreutils sha256sum and Perl shasum both read checksum lines from stdin when the operand is - or when there is no operand. The hashing path already had that StdinOperand treatment (PR #74); check mode did not.

User-visible behavior

# before
$ printf 'HASH  /f\n' | sha256sum -c -
# => rc=1, sha256sum: -: No such file or directory

$ printf 'HASH  /f\n' | sha256sum -c
# => rc=0, no output

# after (matches GNU/BSD)
$ printf 'HASH  /f\n' | sha256sum -c -
# => rc=0, /f: OK

$ printf 'HASH  /f\n' | sha256sum -c
# => rc=0, /f: OK

A checksum file operand is unchanged: sha256sum -c /sums still verifies that file. A mismatch still prints FAILED and exits 1. JustBash.exec/2 does not raise.

Changes

• Check mode uses defaults_to_stdin/1, so a missing operand is -
• Check mode reads the checksum list through StdinOperand.read/4, so - is stdin rather than a path
• Tests cover - as stdin, a missing operand, a normal file operand, and a mismatch via stdin — for both sha256sum and shasum

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Testing

• Added new tests
• All existing tests pass
• Checked against GNU sha256sum 9.4 and Perl shasum 6.04

Local gates:

mix compile --warnings-as-errors
mix format --check-formatted
mix credo --strict   # only the pre-existing apply/2 test fixture
mix test
# Finished in 32.5 seconds
# 2 doctests, 62 properties, 5364 tests, 0 failures (5 excluded)
mix docs --warnings-as-errors
mix dialyzer
# Total errors: 13, Skipped: 13, Unnecessary Skips: 0

Checklist

• My code follows the style guidelines of this project
• I have run mix format
• I have run mix credo and addressed any issues
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes