fix(arcup): accept checksum files without a trailing newline

[osr21]

What

verify_checksum_file() decided whether a checksum file was empty from the exit status of:

read -r expected_checksum expected_name < "$checksum_path"

read returns a non-zero status when it reaches EOF before encountering a newline — even though it has already assigned the variables. So a .sha256 file whose single line has no trailing newline (valid, and produced by several non-GNU checksum tools or by manual editing) was rejected with Checksum file is empty, aborting an otherwise-correct install.

Fix

Decide emptiness from the parsed hash rather than from read's exit status:

read -r expected_checksum expected_name < "$checksum_path" || true
if [[ -z "$expected_checksum" ]]; then
    error "Checksum file is empty: $checksum_path"
fi

A genuinely empty file still produces an empty $expected_checksum and is still rejected, so the empty-file guard is preserved.

Repro

printf '%s  archive.tar.gz' "$(sha256sum archive.tar.gz | cut -d' ' -f1)" > archive.tar.gz.sha256  # no trailing newline
# before: aborts with "Checksum file is empty"
# after:  verifies normally

Tests

Extends test_checksum_validation in arcup/test_arcup.sh with two cases:

• a valid checksum file without a trailing newline now passes;
• a genuinely empty checksum file is still rejected.

Both fail on the current arcup and pass with this change. Full bash arcup/test_arcup.sh suite is green.

Notes

Current release .sha256 assets do end in a newline, so this is a latent-robustness fix in the checksum-verification path rather than an active break — but arcup shouldn't refuse a checksum file it verifies correctly simply because of a missing trailing newline.