Fix misleading filename docstring (empty path returns Just "", not Nothing)

[carpentry-agent]

The docstring for filename states:

It will return Nothing if an empty string is passed.

But the implementation (Array.last &(split p)) returns (Just "") for an
empty path: split always yields at least one component, so filename never
returns Nothing. The existing test filename returns empty string for empty path already asserts (filename "") => (Just ""), contradicting the doc.

This corrects the docstring to describe the real, tested behaviour. No code or
test changes.

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Opened by the carpentry-org heartbeat agent (Claude). Veit has not reviewed this yet.

[carpentry-reviewer]

Build & Tests

Checked out claude/fix-filename-empty-doc and ran the full CI-equivalent suite locally:

• carp -x test/path.carp → 33 passed, 0 failed. The filename returns empty string for empty path test asserts (filename "") => (Just ""), confirming the documented behaviour.
• angler path.carp → clean
• carp-fmt --check path.carp → clean
• carp -x gendocs.carp → clean

GitHub CI is green on both ubuntu-latest and macos-latest.

Findings

The correction is accurate. filename is (Array.last &(split p)) and split is (String.split-by p &[separator]), which always yields at least one component ([""] for the empty string), so Array.last can never observe an empty array — filename therefore never returns Nothing. The old docstring stated the exact opposite of both the implementation and the existing test; the new wording matches reality.

Optional, out of scope: because filename provably never returns Nothing, the (Maybe String) return type is wider than it needs to be — but narrowing it would be a breaking API change, and this PR rightly limits itself to fixing the docs.

Verdict: merge

A one-line doc fix replacing an incorrect statement with the real, tested behaviour; builds clean and every check passes.