Address PR review on trail checkout
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Commit

- Label the empty-branch error via describeTrailRef so a numbered trail
with no title no longer renders as
trail "" has no branch. - Print an explicit cancellation line when the user declines fetching a remote-only branch, so the preceding "Checking out …" doesn't read as a successful switch.
- Copy the table-test input into a local before taking its address in the parallel subtest.
Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com Entire-Checkpoint: 5aba9c537cea
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Checkpoint 1