Make the demo output newline contract explicit

[Fiddlehead-MB]

The observed failure

The built-in demo says each output file must contain exact text, but its checks use shell command substitution:

test "$(cat alpha.txt 2>/dev/null)" = "alpha ready"

Command substitution removes trailing newline bytes. As a result, the current check cannot distinguish alpha ready with no newline, one newline, or multiple trailing newlines. It can report PASS for outputs that are not byte-for-byte identical.

The proposed fix

• Propose one LF as the explicit ending for each one-line demo output.
• Compare the expected bytes using printf ... | diff -u - file.
• Keep the README example on the same contract.
• Add regression coverage for the proposed contract.

Proposing one LF for these one-line files — happy to match a different convention if the project prefers.

The change is limited to the built-in demo, the README example, and one test. It does not change worker invocation or general check execution.

Verification

alpha.txt bytes	Expected result
b"alpha ready"	FAIL
b"alpha ready\n"	PASS
b"alpha ready\n\n"	FAIL
b"alpha ready."	FAIL
File missing	FAIL

Executed proof

RINGER_NO_SELF_UPDATE=1 python3 -m unittest discover -s tests

All 254 tests pass on Python 3.12.3.

Related: #31 touches the demo wording but doesn't change the comparison logic — this fix is independent of it. Happy to rebase if #31 lands first.